Classic Cartoons:
* A Bug's Life: Flik, Atta, Hopper
* Alice in Wonderland: Alice, The Cheshire Cat
* Astro Boy: Astro Boy
* The Adventures of Tintin: Tintin, Captain Haddock
Modern Cartoons:
* Adventure Time: Finn, Jake, Princess Bubblegum
* The Amazing World of Gumball: Gumball, Darwin, Anais
* Amphibia: Anne Boonchuy, Sprig Plantar, Polly Plantar
* Avatar: The Last Airbender: Aang, Katara, Sokka
* Arthur: Arthur Read, D.W. Read
* Aqua Teen Hunger Force: Master Shake, Frylock, Meatwad
Disney Characters:
* Aladdin: Aladdin, Jasmine, Abu
* Ariel (The Little Mermaid): Ariel, Flounder, Sebastian
* Anna and Elsa (Frozen): Anna, Elsa, Olaf
* Alice (Alice in Wonderland): Alice, The Mad Hatter, The Cheshire Cat
Other:
* Adventure Time: Finn, Jake, Princess Bubblegum
* Angry Birds: Red, Chuck, Bomb
* Archer: Sterling Archer, Lana Kane, Cyril Figgis
* Asterix and Obelix: Asterix, Obelix, Getafix
This is just a small sample. There are many more cartoon characters that start with the letter "A"!