Derived from the wiki:

"Cosmo is great in the kitchen! He loves baking sweet treats for all his friends! He is often seen with his best friend Sprout! Cosmo inspires children to have generosity."

Cosmo is a helpful, yet cautious Toon. He displays a sweet and caring personality. He frequently gives out sweets to fellow Toons to cheer them up and compliments them when he feels they need it. However, he appears to be rather anxious as well. Cosmo’s hobby of baking can be assumed to stem as both a source of comfort and a desire to help others.

He is clueless on some jokes and can be rather gullible to teasing, but he always makes sure to care for his friends and encourage them when they most need it. It should be noted that though Cosmo can be rather soft-spoken and anxious, he is by no means a pushover. Cosmo's sensitive side is mentioned in Flutter and Sprout's conversation, as Sprout mentions Cosmo being in tears for a week when the gumball eyes melted on the cookies they made of Flutter's face.

He should have a soft, masculine voice. No particular rules, try what you feel would fit him best!