Voiced by: Evan McGuire (child); Micheal McGrath (adult)
- A Boy and His X: A Boy and His Cat. Forms a close bond with Pangur-Ban.
- Friend to All Living Things: There's the whole goose scene where he's introduced, where he's very gentle with it once it's caught. He also quickly makes friends with Pangur Ban the cat and later Aisling, a forest sprite.
- He Is All Grown Up: And has completed the Book of Kells.
- The Hero: The main character of the film.
- Innocent Blue Eyes: To show his naiveté (especially from his uncle's POV) and his curious nature.
- Nephewism: When Brendan was very young, his parents died in a Viking attack. His mother's brother, Cellach, managed to save Brendan from the same grisly fate, and grafted him into the Abbey as a brother.
- Nice Guy: Brendan is a polite, honest, and respectful young boy.
- Prophetic Dreams: A couple of times throughout the movie. Most notably, he dreamed of Iona being sacked by Vikings the night before Brother Aidan arrived and brought news that Iona has fallen to the Northmen. He also dreamed of the Eye of Crom Cruach before he'd even heard of its existence and of being attacked by black tendrils in a fashion similar to how Aisling was later when he went into the cave.
- Protagonist Title: In the alternative title — "Brendan and the Secret of Kells".
- Red-Headed Hero: Brendan is the hero and has red hair.
Voiced by: Christen Mooney
- Animal Motifs: Wolves. It's her alternate form and she's The Leader of a wolf pack.
- Animorphism: One of her abilities is being able to turn into animals. She mainly turns into a wolf, but the opening shows her as a salmon and a deer as well.
- Breakout Character: She pretty much stole the audience even with the limited screentime she had. Aisling even found her way into Empire's list of 50 best animated movie characters. She also had the honor of going to the Oscars in 2010 when the film was nominated for Best Animated Feature.
- The Fair Folk: She's nicer than most examples, but she still isn't above siccing her wolves on people who trespass in the forest, which she considers hers.
- Friend to All Living Things: She states that the forest is hers, and it's implied that she can communicate with everything in it. She later forms a close friendship with both Brendan and Pangur Ban.
- Jerk with a Heart of Gold: She's more than a bit prickly towards Brendan at first, initially setting her wolves on him and then demanding that he leave her forest, but she soon warms up to him and becomes quite loyal to him afterwards.
- Meaningful Name: Aisling has a meaningful name in more than one way: not only is it an Irish girl's name meaning "dream" or "vision", it's also a kind of Irish poetry that frequently features a woman who represents Ireland itself, while calling back to its previous pagan beliefs. In the film, Aisling serves the same purpose.
- Morphic Resonance: Whenever she changes shape, you can tell it's her thanks to her white colouring and green eyes.
- Mystical White Hair: She has white hair and, being of The Fair Folk, is definitely magical. According to the tie-in comic, it's a racial trait of all her people.
- Rapunzel Hair: That reaches all the way to her ankles.
- Really 700 Years Old: She states outright in the opening that she's "lived through many ages", and decades later, when Brendan is an adult, she looks exactly the same as she did when he was a child.
- Super Strength: This petite girl can lift and/or push down statues with ease.
Voiced by: Brendan Glesson
- Bald of Awesome: In the comic prologue, he overpowers and kills a trio of vikings to save Brendan.
- Control Freak: He's so paranoid about the Northmen that he's overhauled all normal functions of the Abbey in favor of building a defensive wall. Not even the benches in the scriptorium are free from his tight control; when Aidan moves in, Cellach rearranges to make room for his work, but refuses to let Aidan have any say in the layout of his work space. It wouldn't be so bad if his ideas were sound, but often times he ignores good points brought up by others simply because they're being insubordinate and pulls rank to make them stop talking.Cellach: "The scriptorium will be arranged according to my instructions."Aidan: "But it's not a good—"Cellach: "It will be as I say."
- Cool Uncle: Averted. The Abbott isn't a horrible person, but not exactly "Mr. Warm" even to his own nephew.
- Evil Uncle: Also averted, though he is the movie's antagonist for a good portion of it.
- Jerk with a Heart of Gold: The Abbott is a stern, no-nonsense man with a stubborn disposition, but end the end he does mean well.
- Papa Wolf: He killed vikings to protect his nephew.
- Perpetual Frowner: Never see him smile once. Until Brendan's return.
- Well-Intentioned Extremist: His actions aren't ideal, but his motives are good.
Voiced by: Mick Lally
- The Ace: He was considered to be the perfect illuminator, largely due to the fact that he was the Keeper of the Eye of Crom.
- A Boy and His X: A Man and His Cat. Pangur-Ban was his constant companion throughout his travels.
- Cool Old Guy: A lot more warmer than the Abbott.
- Man Child: Aidan has clearly not let go of his childish side, despite the fact that he carries the immense burden of loss and responsibility.
- Nice Guy: Aidan is easy going, kind, warm, and optimistic.
Voiced by: N/A
- Cats Are Snarkers: Of the Silent Snarker variety.
- Defrosting Ice Queen: She really starts to warm off Brendan. Eventually, after Aidan's death, she now travels with an adult Brendan.
- Meaningful Name: Pangur-Ban is named after the oldest known poem written in the Irish language...which is about a cat.
- The Shadow Knows: Even when transformed into a sprite by Aisling, her shadow remains that of a normal cat.