Kevin Tremain a.k.a. Post is relatively unknown to most, other than his association with Onslaught and his encounter with the X-Men.
POWERS: According to Marvel, Kevin was born a mutant with super strength, durability, as well being highly intelligent. Post had several armored detachable plates on his body. They allowed him to cloak and teleport. they could act as scanners, form energy blasters, and counteract another mutant's power. For example, after an attack by Iceman, Post conducted heat that traveled through the ice to Iceman's body. Post connected himself to the surrounding area when confronted by the X-Men in X-Men #50. Onslaught also provided Post with a psionic screen preventing others from reading his mind and hiding information about Onslaught. Tremain could also telepathically communicate to Cable.
Tremain was captured and studied by the Mandarin to exploit Kevin's abilities for his own diabolical purposes. On a secret mission, the Six Pack attacked the secret base and stumbled acrossTremain. Tremain had severe injuries, and Cable tried to save him in the process. Cable used his telekinesis to keep Tremain's body together, but inevitably was forced to give him a blood transfusion. Although Tremain survived this trauma, Cable was under the belief that Tremain had later died.
Years later, Tremain resurfaced as Onslaught's herald Post. In X-Men #50, Onslaught forced Gateway to teleport Cyclops, Storm, Iceman, and Wolverine to where Post could 'test' them to see if they were worthy of defeating HIM. Through their collective abilities and teamwork, the four managed to defeat Post and were transported back to the Mansion. Barely surviving and badly bruised, the four of them lived to fight another day.
Some time much later, Post attacked Cable. Cable instantly recognized him as Tremain and they fought. Post defeated Cable and left him for the Hulk to finish off. Post was later charged with ensuring the Sentinels would launch, and did so. He and his master Onslaught were eventually defeated by the heroes.
In Cable #87, Post met his untimely demise when he attempted to assasinate Senator Robert Kelly, and was thwarted by Pyro in a last ditch effort to redeem his past aggressions as a Brotherhood member and save Xavier's dream of mutant and human prosperity.