|Cartoon debut||"Dear Adil"|
|Voiced by||Evan Smirnow|
Onder is Adil's best friend and classmate.
Onder is a cat with a tan skin complexion, he wears a long-sleeved orange shirt with light yellow stripes and a white collar. His pants are a dark grey and his hair is a chocolate-brown color.
Very similar to Buster, Arthur's best friend, Onder struggles in finishing math assignments and has quite an imagination, such as when he thought that his teacher would throw him into the Sea of Marmara for not finishing his homework. He also admits that he can't swim.