Cas is a beaver. His outfit is an open orange puffy vest and a pair of denim hot pants (which appears to possess a large bulge), with extra thick round glasses over his eyes.
He lives on the other side of the Southeast Beach in a log cabin, which is full of dakimakura–or body pillows–of Isadora, who himself appears to be a slim-looking lizard with a huge cock. He's most likely an otaku, given that he is quick to fluster, has a range of posters and aforementioned body pillows, and has a cliché nerdy look to him.
He helps out Joe by lending him his “Ass Masters” book, enabling him to slam downwards with his bare-naked rump.
Cas (Castor) has a brother named Lux (Pollux). Also, Ket seems to know Cas, so it is possible they are friends.