Pretty positive that Trigger was created with the Master's DNA.
[/theory] I know I don't have any proof, but I always had this feeling that the Master was Zero and X in a human form. I have little to back it up, but just a hunch. Like, since the Reploids and humans literally started to be able to cross with each other, when X and Zero were cyberelfs they had the ability to be human at one point, and their bond of friendship led them to be one body. (Cheesy, almost sounds like a cry for a fan-fic! Bleh!)
PURE speculation, but it would help to prove why Mega Man Trigger the purifier unit resembles and acts like previous Mega Man incarnations.
(Another bothersome issues is when i found out that his name was Rock Volnutt, and not Megaman Volnutt kinda made me mad. Because when your told "Your real name is Mega Man Trigger, a purifier unit, like other Mega Man." [seen towards the end of ZX, that the term 'mega man' was starting to shape into a 'class' of sorts] I wouldn't be surprised if I heard my real name was the same thing, but different last name. Actually, I would have probably been annoyed/confused as to why this "Juno" guy has the same name I did before he tried to tell me my real name.)
Rock Volnutt to Rock Man Trigger sounds like a bigger name difference than Mega Man Volnut to Mega Man Trigger.
Speaking of theories and Triggers past, this person made a neat theory based on the little information given throughout the games=
http://androidarts.com/mml/reaverbot_legends.htmNot sure what the site was about, but it was an interesting read, and he tried to fill in the gaps. Its a good theory, but, its not official.