Krostofferson is an almost off-white colored fox that usually wears very kept clothes that are usually blue. We see him in the film in mostly a blue collard shirt with very pale white shorts, high socks, and red shoes.
He has blue eyes, resembling Mrs. Fox in this way. He also is rather tall for his age, even mentioned by Mr. Fox to be "bigger for his age." He has a lean body type.
He is a rather mild-mannered fox, not really seen yelling in the movie. He is very considerate and is not confrontational unless someone is being offended, in which case he will step in. He is very loyal toward his friends and family.
Kristofferson is extremely skilled in many areas of expertise, many of the characters dubbing him a "natural" with whatever he tries. He knows karate and is shown to be very skilled in it, in one instance using it to defend his cousin.
Kristofferson is first seen as he arrives on a hill with the label "Unaccompanied Minor." He is the Fox family's nephew from Mrs. Fox's side of the family, coming to stay with them as his dad has double pneumonia (his mother is not mentioned in the film).
The next time we see him is when he and Ash are diving from the tree into a small pool and showed magnificent skills in diving with grace. Mr. Fox clapped for him and stated that "this kid is a natural!" He then proceeds to meditate, the Fox family very confused at first by his actions.
Later he is conversing with Ash if he can move his spot to sleep somewhere more comfortable, to which Ash snaps at him and tells him that he's annoyed with his sad houseguest routine. Kristofferson deals with his bed situation, but starts crying as soon as he lies down. Ash notices and checks on Kristofferson after hearing the noise. He then turns on his train set, which calms down Kristofferson. The both of them watch the train go around and around.
At Ash's school, Kristofferson has enrolled and managed to distract Ash's lab partner, Agnes, who has seemed to develop a crush on Kristofferson and vice versa. The bully of the school asks Kristofferson why his cousin is "such a wet sandwhich" and Kristofferson tells him off by saying that he sounds like a bully. After this small conversation, the same bully causes their experiment to blow up, making Kristofferson slightly annoyed and he goes to extinguish the Magnesium. Agnes is very impressed with this and tells him that she likes his ears, to which he replies that he likes her spots.
Later Kristofferson is shown the ropes to a game called Whack-Bat. He listens to the coach explain it quickly and subsitutes for Ash. He plays and shows a firm understanding and skill of the game. The coach is rather impressed and comments to Ash, stating that "he really is your father's nephew." Ash replies that he wasn't related by blood. He's from his mother's side. After this, he sees Agnes holding up a K! and he asks what it stands for, rather shocked at it. She tells him that it's for pep, but he rebuttles with "... it's a K." Agnes shrugs and tells him "we're going steady," implying her new relationship with Kristofferson.
Kristofferson is later seen accompaning Mr. Fox and Kylie to go steal Bean's cider. He tells his uncle that he's honored to have been invited but he doesn't like being dishonest to people, but Mr. Fox assures him that if doesn't say anything than that's technically not lying. He later helps trap the Rat of security in Bean's cellar by pulling a rug from under him and locking him in a crate. After this, he remarks about Mrs. Bean's bad eyesight and that's how she must not have seen him. When Mr. Fox tries to sneak out again, only to be stopped by Mrs. Fox, Kristofferson is seen about to go to with his bandit hat on, but Mr. Fox says that he's not with them and sent him to bed.
After Mr. Fox's tail was shot off, Ash rants and eventually says at least Mr. Fox's injury wasn't as bad as Kristofferson's dad who had "one foot in the grave and one on a banana peel." Kristofferson throws an acorn in frustration, it landing perfectly in a cup he was holding. He states that he will go meditate for half an hour, showing that he was rather upset and angered by Ash's comment as well as his worry for his dad. Ash is told that he has 29 minutes to think up a proper apology.
When the house is later dug up and the Fox's are on the run, Kristofferson confronts Ash about the rumors he was spreading about him (and Ash denies everything) and tells him that he doesn't appreciate it. When the foxes (plus Kylie) meet up with the other animals, Ash is bullied by Beaver's son and Kristofferson stands up for him by beating up the beaver. He seemed to realize that Ash was extremely self-conscience and even vulnerable. Ash tells him, "I can fight my own battles" to which Kristofferson replies with, "No you can't."
Some time after Mr. Fox leads the party of animals to steal everything from the farmers, Ash convinces Kristofferson to steal back his dad's tail and he agrees after a while. He goes with Ash to retrieve it, only to be taken by surprise by Mrs. Bean and captured. He is held for ransom and put in a crate on top of a box in the attic of Bean's house.
After Ash rescues him from the attic, the two join up with Kylie and Mr. Fox and escape. As they ride away, Kristofferson also sees the wolf with the rest of them.
At the end of the movie, Kristofferson is still seen with the Fox's and even with Agnes as they climb through the pothole leads to a supermarket. He dances with the rest of the cast as the movie ends.