Ash is a male red fox. He has his mother's light fur colour and his dad's hazel eyes, although none of his parents have his distinctive black markings around his eyes. Ash is short for his age (which also is a trait that neither of his parents share), and is mocked for this by bullies, which contributes to his cynical disposition. He dresses in a superhero-esque manner (based on "White Cape", a comic book character whom Ash idolizes) wearing a white fleece jumper, white pants tucked into his socks and another jumper which appears to be a mock cape.
Ash is twelve fox-years of age when he first appears, being called in by his parents to prepare for school. Ash makes a sorry attempt to feign sickness, then bluntly says he just doesn't want to attend class. His mother casually brushes it aside and tells him to get ready, which he obeys. He appears dressed like a favorite superhero, and rudely spits on the floor when Mr. Fox questions his apparel. Ash is further annoyed by the news that his cousin Kristofferson is staying with the family for a time, to which Ash refuses to share his roomspace.
No sooner do the two cousins get together, they start off with Ash being overshadowed by Kristofferson's ability to excel him in everything. Ash is further embittered by the fact that Kristofferson was invited to accompany his father on a cider raid, while Ash was strictly refused. Ash's relation with his mother is only slightly better, being scolded twice for his apathy towards Kristofferson's hospitalized father. Fuming, he accuses his family of thinking him incompetent and inept. When the family evades capture by Boggis, Bunce, and Bean by digging deep into the earth, Ash and Kristofferson have a brief spat about school gossip and personal differences.As the other animals arrive to reprimand Mr. Fox for endangering everyone, Ash is bullied by Mr. Beaver's son, but Kristofferson defends him with a karate kick to the beaver's nose. Ash remains unimpressed, claiming he can stand up for himself, though he does slowly consider his cousin's abilities as useful. Feeling inspired to prove himself to his parents, Ash enlists Kristofferson to join him into stealing back Mr. Fox's tail, which Mr. Bean was wearing as a neck tie. The mission fails as Ash barely escapes from Mrs. Bean and Kristofferson is captured.
When Mr. Fox offers himself in exchange for Kristofferson's release, he relates to Ash of the time when he first heard that his wife Felicity was pregnant. Having escaped from a fox trap, all Mr. Fox could ponder over was who his son (or daughter, since his child's gender was unknown at the time) would be, and he's grateful for Ash being his son. Ash begins to cry as his father leaves. Shortly thereafter, Rat arrives with a ranson notice, incorrectly identifying Kristofferson as Mr. Fox's son, to which Ash says that the farmers took the wrong fox. Rat easily recognizes the error and is about to capture Ash, when Mrs. Fox steps in with a chain to defend her son. She is easily overpowered and Ash is stuffed into a sack, until his father arrives to defeat Rat.After organizing a rescue plan for Kristofferson, Mr. Fox requests that Ash and the other children take care of janitorial needs, to which Ash spits in disapproval. He sneaks aboard a motorbike and sidecar to join Mr. Fox and Kylie as they drive to Bean Annex, where Kristofferson is being held captive. Upon arriving at the annex, Ash's small size allows him to climb down inside with a shoelace rope. With Kristofferson in a cage with a thick padlock, Ash asks for a karate lesson, hoping to chop the padlock off with his bare paw. Instead, he clumsily rolls into the cage and knocks it off the edge. With Kristofferson a little fazed from the impact, Ash apologizes for being rude earlier and confessing to being different. His cousin gently forgives him, saying it's okay.
Mr. Fox and his family are soon pinned down by gunfire from an ambush set up by Boggis, Bunce and Bean. Taking in some philosophy from Kristofferson's quick karate lesson, Ash bravely dodges a hailstorm of bullets. He quickly constructs a makeshift whack-bat from a garden rake and uses it to open a gate, unleashing a rabid beagle on the farmers. His father is extremely pleased with his pure animal instincts and declares him an athlete, which brings a smile to Ash's face. Rewarded with a star-embroided bandit hat, Ash and his family escape to safety while the farmers are left behind, trapped by the rabid beagle.
Three days later, Ash and Kristofferson have established a strong friendship, sharing in meditation sessions together with Agnes.
For the majority of the film, Ash is generally in a foul mood. He's grumpy, cynical and ill-tempered. He tries to excel in athletics to be like his father, but either fails or is denied the opportunity. He gets jealous of his cousin Kristofferson, who is better than Ash at almost everything he can do. He takes it personally when his father appears to neglect him and instead pay attention to Kristofferson. He is generally interpreted as a misunderstood character who tries to be normal or someone who he is not, which leads him to be depressed and bitter.
He gradually develops into a more considerate character. He begins to reconcile with Kristofferson after the latter defends him from a bully. His mother also confides in him that being different is not bad, though he is not wholy convinced of this. He later develops enough trust in Kristofferson to invite him into stealing back his father's tail. When Kristofferson is captured by Mrs. Bean, Ash takes all the blame with humility. Ash is humbled to tears when his father decides to surrender himself to the farmers in exchange for the lives of all the animals and Kristofferson's release. Upon rescuing his cousin, Ash apologizes for his previous rude behavior, to which Kristofferson kindly forgives him, establishing a friendship between them. Ash's confidence reaches its peak after he single-handedly provides a distraction to allow him and his family to escape Bean Annex, earning his father's most heartfelt praise. Ash takes up meditation sessions with Kristofferson everafter, having become good friends.
- Ash is voiced by actor/musician Jason Schwartzman, a regular in Wes Anderson's films, including Rushmore, Hotel Chevalier, The Darjeeling Limited, Moonrise Kingdom, Castello Cavalcanti, The Grand Bupadest Hotel, She's Funny That Way, and Isle of Dogs.
- The lower half of Ash's body was wrapped in cling film during shooting.
- Like his mother, Ash appears to be ambidextrous, using both his left and right hand dominantly at random times throughout the film. This is most likely an oversight, though.
- His favorite music includes "Let Her Dance" by The Bobby Fuller Four.
- In the original book, Mr. and Mrs. Fox have four children, none of whom are given specific names.