Jack's sister: [sees ice cracking beneath her feet] Jack, I am scared.
Jack: I know, I know... But you're gonna be all right; you're not gonna fall in. We're gonna have a little fun instead.
After being tossed into a ravine at the South Pole by Pitch, Baby Tooth shows Jack that he still has his Tooth Box that he took from Pitch's Lair. It shows him the memory of him and his little sister, on a frozen pond, and that she was on ice that was about to crack. He saved her by making a game out of hopping off the thin ice, then sacrificing his own life to get her to a safe spot. It was shown that he loved his sister very much, shown not only with this selfless act, but because, once his memory returned, he was only happy and grateful to know that she had survived, rather than be disheartened that he had drowned.
Her last appearance was when Jamie told Jack he was scared, causing Jack to remember a similar dialogue with his sister on the ice and making him realize his center is Fun.
Jack's mother: Be careful.
Jack: [laughs] We will.
After Jack opened his Tooth Box, she appears in his memories giving him advice as Jack ran toward the forest. She also appears telling him and his sister to be careful, which shows her caring side but also that she's confident that Jack is a responsible son and can take care of his little sister.
"You can see me?" - Jack Frost to Jamie.
Of all the children Jack had ever encountered, he would find himself drawn to one particular child who truly believed in the Guardians; a boy by the name of Jamie. Jack used his winter magic to give Jamie and his friends a Snow Day and guided Jamie as he rode his sled on an epic ride, ending with Jamie losing a tooth. The result made Jamie excited for a chance to see the Tooth Fairy, which frustrated Jack, as he'd hoped the Snow Day and sled ride he'd given Jamie and his friends would get them to believe in him. Jack later observed Jamie with his mother and sister as he told the story of his epic sled ride and was shown to be upset by his lack of recognition for his role in it.
When the Guardians collected the children's teeth themselves, Jack went to Jamie's house with Tooth to collect Jamie's tooth and was shown to be fond of the boy despite Jamie being unable to see him. Jack got gently admonished for his role in Jamie losing his tooth in the first place and was shown to be embarrassed by it when confronted. Jack also used collecting Jamie's tooth to show Tooth just how different it is to work with kids up close rather than from a distance as the other Guardians do. Jamie eventually woke up due to North's antics and Jack was excited when he realized the boy could see the Guardians until he realized that Jamie still couldn't see him. Despite this, he took the opportunity to prank Bunnymund by setting Jamie's greyhound Abby on him by activating Jamie's alarm clock. This resulted in all but Jack and Sandy being knocked out, but Jamie clearly enjoyed Jack's prank even though he didn't know who was behind it.During the climax of the movie, it was revealed that Jamie was the last child to believe in the Guardians. Jack excitedly rushed to Jamie's house after seeing that he was the last child on Pitch's Globe of Belief. Jack arrived just as Jamie was about to lose belief due to a lack of response from the Easter Bunny through his toy rabbit for a sign. Jack used his powers to draw an easter egg and bunny on the window, restoring Jamie's belief. Jack added a special touch by making the bunny come to life and jump around Jamie's room, but this had an unexpected side-effect: when the bunny exploded into snow above Jamie, he realized that Jack Frost was visiting him and finally started believing in him. Jack was so stunned to hear a child calling his name that he didn't even realize that Jamie was looking directly at him, open-mouthed for a minute. Jack finally realized that Jamie could see him and it nearly drove him to tears to finally have a believer. Jack and Jamie instantly became friends with Jack assuring Jamie of the existence of all the spirits he believes in and Jamie excited when he learns that Jack was responsible for the sled ride he had. Jamie even acknowledged Jack's presence to his mother when she asked who he was talking to. When the other Guardians arrived, Jamie followed Jack outside to meet with them and in the process, revealed that he could now see Jack, earning him a playful nudge from Jack. He also helped settle the differences between Jack and Bunnymund when he revealed that Jack made him believe in Bunnymund when he was on the verge of losing his belief, deeply touching the Easter Bunny. When Jack was knocked out of the sky by Pitch, Jamie was left greatly concerned for him and when surrounded by Nightmares, told Jack he was scared. Going to comfort Jamie caused Jack to realize that Fun was his center and he made Jamie laugh despite the danger by hitting Pitch with a snowball. When the Guardians collected Jamie's friends, Jack allowed Jamie to fly with him, carrying him to each of his friends' windows to wake them. Seeing Jamie like this and Jack's magic got him even more believers. When the Guardians were faced with the apparently unstoppable Pitch, Jamie stood with Jack, the only one left to have any real power and was the first to declare that he'd stand with the Guardians. His power of belief turned Pitch's Black Sand back into Dream Sand and restored Sandy, saving Jack from Pitch's attack. After Pitch was defeated, Jack started a friendly snowball fight by pelting Jamie with a snowball. Jamie attended Jack's Guardian Ceremony and Jack looked to him for approval before agreeing which Jamie proudly gave him. When the Guardians started to leave Jamie, having grown close with Jack, got worried that if he never saw him again he'd lose belief in his new friend and the other Guardians. Jack reassured him that the Guardians would always be there for him and as they would always be in his heart too, it made him something of a Guardian himself. Jack also reminded him that just because you can't see something all the time doesn't mean it doesn't exist. As Jack left, Jamie gave him a hug, shocking Jack who wasn't used to children hugging him but who quickly returned it and showing the bond of brotherhood that had developed between them.
"Hey, little Baby Tooth; you okay?" - Jack Frost to Baby Tooth.
Baby Tooth is one of Tooth's mini fairies that follows her where ever she goes. However, unlike all the other fairies, Baby Tooth doesn't have a blue feather sticking from her forehead. Instead she has a golden feather, just like Tooth. Jack saves her from being taken prisoner by one of Pitch's nightmares. Afterwards, he instantly gives her the name "Baby Tooth" after seeing her gold feather. His naming her was the start to their new friendship. Just like Tooth, Jack is protective of Baby Tooth and he doesn't mind her following him around from time to time.
In Antarctica, Jack's protectiveness for her - and his selfless character - was put to the test when Pitch gave him a choice between Baby Tooth and his staff. Jack, as expected, chose the former, and had to bear pain due to the staff being broken into two by Pitch. There, Baby Tooth, seeing the distressed Jack, decided to teach Jack how to use the Tooth Box. As a result, he was able to remember parts of his past and discover that he's a Guardian. This was done in order to make him remember happier times, so that he wouldn't lose hope, and could continue; Baby Tooth fulfilled her job as a mini-Tooth Fairy, as well as a friend to Jack.
Jack: "If you're not a kangaroo, then what are you?"
Bunnymund: "I'm a bunny. The Easter Bunny. People believe in me."
Jack and Bunnymund have a long rivalry with each other that presumably began in '68 when Jack caused a blizzard on Easter Sunday, resulting in Bunny holding a grudge against him ever since. Bunnymund was the only one who objected to bringing Jack into the fold of the Guardians and Jack never wasted a chance to irritate or tease Bunnymund, who in turn rubbed salt into Jack's emotional wound of children not believing in him. Though it can be said that Bunny never appreciated others hurting Jack in that way - his immediate reaction to Pitch's hurtful words directed at Jack is an evidence. They were competitive towards each other, as was seen during the Guardians collecting the teeth (with Jack blasting Bunnymund with frost, and Bunnymund making Jack fall into one of his holes). However, Jack helped all of the Guardians who, despite protecting children, never spent time with them, to become closer to them, particularly for Bunnymund who grew a soft spot for little Sophie, Jamie's younger sister.
Bunnymund was touched to learn that Jack helped Jamie continue to believe in him, and they finally became true friends as Bunnymund watched in pride and gratitude when the Guardians officially welcomed Jack into their fold after Pitch's defeat.
North: I hope the Yetis treated you well.
Jack: [sarcastically] Yeah, I love being shoved in a sack and tossed through a magic portal!
North: Oh, good. That was my idea!
Jack and North are friendly towards each other, possibly due to the fact they both (on different levels) represent Christmas and Winter. Although, Jack's sarcasm is sometimes lost on North. Out of all the Guardians, North shows the most faith in Jack, being very supportive of him and teaching him about what every Guardian must discover for him or herself: their center. North told Jack that his own center was bringing Wonder to the world. This lesson, along with Jamie, eventually helped Jack realize his center was Fun.
When Jack is chosen, North is more contemplative than anything else and despite Bunnymund's objections, calls Jack a Guardian. He also comes up with the idea to kidnap Jack with a sack and is proud of himself for it, not catching Jack's sarcasm about his not liking it. When Jack asks about being on the Naughty List, North gets slightly stern when telling him that Jack holds the record but is still willing to wipe the slate clean due to Jack's new status. He is also amused when Jack tells him that he has repeatedly tried to break into North's Workshop, something that is clearly a surprise to North. North is the one who gets Jack to agree to help them, first by seeing his impressive sleigh and then promising to help Jack get his memories back. He quickly befriends the young spirit and when he's disheartened by his failure to save Sandy, tells him that Sandy would be proud that he saved the other Guardians and that while he may not know who Jack used to be, he knows that in this life he's a Guardian. His words seem to reach Jack as he looks less depressed after hearing them. When Jack apparently betrays them on Easter however, North is the first to turn against him, accusing Jack of betraying them to Pitch and turning his back on the winter spirit. In return, Jack drops the nesting doll North gave him before leaving. When Jack returns to help at Jamie's house, North is stunned and slightly suspicious to see him but this disappears when Jack reveals that he came to help Jamie keep believing and Jamie comes out of the house. When its revealed that Jamie can now see Jack, North is stunned but is very happy for his friend. Jack and North share a moment of amusement at Bunnymund's state and Jamie's reaction to it. He and Jack fight side by side in the following battle and when Jack starts a snowball fight between the kids, the Guardians and their helpers, North realizes that Jack has found his center and gives him a nesting doll of it, indicating North may have already figured it out before he's drawn into the snowball fight. Jack has great respect for North and North has even shown himself to be a father-figure to Jack.
Sandy: [throws Pitch off a rooftop after repeatedly slamming him on tops of some buildings]
Jack: [looks on, astonished] Remind me not to get on your bad side.
Jack and Sandy share a good friendship with respect for one another, and it is implied they've been friends since before the movie's events. They fought side by side against Pitch and his Nightmares. When Sandy was apparently killed by Pitch, Jack was devastated but remained to fight the good fight to honor Sandy. Jack was overjoyed when Sandy was brought back to life by the dreams and belief of the children.
Tooth: Left central incisor knocked out in a freak sledding accident. I wonder how that could have happened. Jack?
Jack: [embarrassed] Kids, huh?
Tooth: [gives Jack an amused smile and leaves a coin under Jamie's pillow] This was always the part I liked most: seeing the kids... Why did I ever stop doing this?
Jack: Hmm. It's a little different up close, huh?
Tooth: Thanks for being here, Jack. [touches his shoulder, looking guilty] I wish I'd known about your memory. I could have helped you.
Jack: Yeah, well... Look, let's just get you taken care of, then it's Pitch's turn.
Toothiana, known as the Tooth Fairy and nicknamed as Tooth by her Guardian friends, is very sweet albeit clumsy towards Jack Frost. She has a crush on Jack; while Tooth is more reserved about her feelings, her minions - as they're extensions of herself, they feel mostly the way she feels - are more obvious about their crush on Jack, what with their fainting and fan-screaming. Tooth has to constantly calm them down due to their obvious behavior. She was the most excited one when the Guardians learned that Jack Frost was chosen as the next Guardian. As soon as she met him, she checked his teeth first, possibly giving Jack a little strange impression of her.After the Tooth Palace was attacked, Jack and Tooth had a proper conversation; during this, Jack's more caring side surfaced as he was concerned for the well-being of Tooth, who also told him that every spirit had memories before they became a spirit; at this, Jack was shocked, since he was apparently the only one unaware of his memories. Also, Tooth was the first one to learn that he didn't have a memory of his life as a human. Tooth wished to help Jack regain his memories, and she said that to him when they were in Jamie's room, collecting his tooth. Tooth - Sandy and North included - was kind and welcoming towards Jack from the start. In the end, she, because she was excited due to things finally turning peaceful, shared a hug with Jack. Tooth, realizing what she had done, got embarrassed, and flew off, while Jack was surprised initially, but enjoyed the action. Jack tends to be caring towards Tooth mostly. He loses his carefree demeanor around her, out of all the Guardians, as was proved after the attack on the Tooth Palace, and his various interactions with her.
"My name is Jack Frost, and I'm a guardian. How do I know that? Because the Moon told me so. So if the Moon tells you something... Believe it." - Jack Frost.
The moon was what Jack first saw when he was resurrected. The Man in the Moon told him his name, never to speak to him again. Jack was seen spending times at night, in the movie, gazing at the Moon, asking questions, but never receiving an answer. This, and the lack of friends and people's belief in him, frustrated him. When he discovered that Man in the Moon told the Guardians that he wanted Jack to join them, he was shocked, for the Man in the Moon never talked to him. But, after the events that carried out, leading him to be the Guardian, he let go of his frustration regarding Manny, and accepted his guidance.
Pitch: What goes better than cold and dark? We can make them believe! We'll give them a world where everything is - everything is -
Jack: Pitch Black?
Initially, Jack and Pitch only just knew of each other, and Pitch ignored him since at the time they first met since Jack was not an official Guardian and took him as a neutral. But as Jack sided with the Guardians and fought against the Boogeyman, Pitch tried to convince Jack it wasn't his fight and insulted him since no one believed in him. He went as far as to subtly arrange for a situation to make it look as if Jack had betrayed the Guardians by giving Pitch Baby Tooth, in exchange for his memories and by making Jack not there to help protect the Warren from the Nightmare's attacks on the Easter eggs, causing the Guardians to ostracize Jack. But then Pitch saw Jack as a kindred spirit, and tried to recruit him, stating what goes together better than Cold and Darkness. Jack refused, as he wanted to be believed in, not feared, so Pitch resorted to blackmail - He demanded Jack's staff (the apparent source of Jack's power) or Pitch would kill Baby Tooth, one of the Tooth Fairy's helpers whom Jack had befriended.
Jack complied and gave Pitch his staff, but Pitch wouldn't give up Baby Tooth (who in rage jabbed his thumb causing Pitch to yell and then throw her across the South Pole). Pitch then broke Jack's staff and the pushed him into an ice crevice while spitefully telling him to be alone. He would later re-face Jack, now a fully-fledged Guardian, representing Fun.