Jermaine is Jake's younger half-brother, the adoptive older brother of Finn, and the son of Joshua and Margaret. Unlike Jake, he has no special stretchy powers of his own, and had this idea built in his head that he wasn't the favorite of his dad compared to Finn and Jake, leading him to be jealous of them. Finally venting his troubles, he lets his childhood home that he's kept preserved burn to waste.
John DiMaggio voices him in his first appearance, with Tom Scharpling voicing him in his second appearance and beyond.
While I like John's work, his performance for Jermaine was more of a throwaway line or two, doing his voice for Jake but pitched down a bit and giving a bit of an urban twang.
While I don't really gel with Tom's tenor pitch for the character, he has the advantage of voicing him more than once. He's pretty funny at the "SSSSSAAAAALT" line, but I feel like this character is pretty much either or. Also he reminds me of Metabad.