Neil

Neil

Unlocked by completing Neil's Challenge

IT Neil

Unlocked by completing IT Neil's Bonus

Zombie Neil

Unlocked by completing Zombie Neil's Bonus

Rastafro

Unlocked by completing Rastafro's Bonus

Afronaut

Unlocked by completing Afronaut's Bonus

Silver Neil

Unlocked by getting a silver medal on all levels

Fresh Neil

Not unlockable

Pitbull

Not unlockable

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