EDIT: Deleted wrong thing I put here, confusion, gawwwwh :?
Anyway afaik we aren't told who he is, all we know is that he and Peter have some kind of affiliation with Group 935 as he asks Peter to find and warn Richtofen and Maxis and he details some info on Element 115. We do know that Group 935 has agents working in America amongst other places thanks to the radio in the little underground hallway bit in the Teleporter C room of Der Riese (iirc it's that one), it's possible they're somehow related - what's interesting is that Maxis mentions that his sources in America say that they have a stock of 115 (hence people expecting an Area 51 map) and the co-ordinates at the beginning of the message for Peter lead to an area in the Nevada desert (where people believe Area 51 to be) and an area in Russia where the Tunguska event happened, a meteor that exploded several kms above ground (this is relevant as we find out 115 is refind from a meteorite).
The man contacting Peter, or rather his group, appear to be performing similar experiments somewhere else outside of Der Riese so maybe they are from another faction of Group 935, the fact that the message doesn't sound German is interesting though, maybe they are Nazi spies or non-German members of Group 935 (or another group affiliated with Group 935 performing similar experiments), other than that though I don't know who the guy that left the message actually is.
Peter is an American spy. He infiltrated group 935 to figure out what they were doing with the zombies and 115 etc.