Commissar Dan is a commissar named Dan (hurr, though his full name is apparantly Daniel Sterne). He has a pet basilisk (Fluffy), and uses it as a frontline tank. Is apparently with the Angry Marines for no other reason than he started tagging along and eventually people just got fed up asking why. The artist behind him is actually a confirmed furry, but that doesn't matter too much when it comes to Dan.
Not to be confused with Commissar Fuklaw, who is
similarly as awesome as Dan so much cooler than Dan.
It is also thought that Dan is in fact an undercover cultist planning to destroy the Angry Marines by killing all the scouts. This will no doubt fail.
The crazy son of a bitch has joined Grendel, Boone and LIIVI in ascended to canon status, on page 116 of the Enemies of the Imperium supplement for Only War. Turns out he was ordering those pig fuckers to ram a Hellbrute! (It went exactly as you can imagine. Badly
Epically at least he's dead).