Jester so far has appeared in the first three Socom games. In the original Socom he was not in Able Element, but in Bravo with well known Spectre (Player 1 in Socom 2 & 3). In both Socom 2 and Socom 3; he is as said, with Spectre (you) in Able. He will always cover you and will never be more than 10 metres away (even if you tell him to!); unlike Wardog in Socom 2 who gets stuck to pretty much everything. His trusty M4A1 will be a key component to contribute towards victory.
According to the original SOCOM's manual, Jester is referred to as Dimone, and Spectre is referred to as Bailey, so it appears that either both Spectre and Jester's callsigns were switched since the first installment (very likely), or the manual simply made a typo (less likely).