Viserys Targaryen is the heir to a shit ton of far away kingdoms but was forced into exile after his father's reign met a bloody end. Jimmy promises to return the Iron Throne to him and thus the two forged an unholy alliance. Viserys served Jimmy as an adviser from episodes 4 to episode 10.
When Jimmy became tired of Viserys' failed plans, he banished the Targaryen to the Ghost Zone.
He is presumed dead.