Kentony

Dr. Kentony

Unlocked by completing Kentony's Challenge

Kentinator

Unlocked by completing Kentinator's Challenge

Clark Kentony

Unlocked by obtaining the "Airtime" achievement

Two-Face Kentony

Unlocked by obtaining the "SUPERHOT" achievement

Uncle Kentony

Unlocked by obtaining the "I want you!" achievement

Golf Kentony

Unlocked by obtaining the "1%" achievement

Carnaval Kentony

Unlocked during Carnaval

Gold Kentony

Unlocked by getting a gold medal on all levels

Skins written in red are not unlockable in-game.
pictures taken from Tom Rutjens's website & in-game