Real Name: Piotr Rasputin (suppposedly... his actual name was never said in the series)
Mutant Power: Ability to change into a organic metal-like form, which gives him superhuman strength and endurance
Voice Actor: Michael Adamthwaite
First Appearance: "Day of Reckoning, Part 1"
Back Story: Apparently, sometime between the episodes "The Cauldron" and "Day of Reckoning", Magneto recruited Colossus onto his team. Colossus' recruitment was not by his own free will, though- Magneto kidnapped Colossus' family and was holding them hostage to get the steel man's cooperation. After Magneto supposedly died, Colossus went back to Russia. He was recruited by the X-Men to help fight against Apocalypse in "Ascension", though, and he is seen in the final X-Men group shot at the end of the 2-parter. Although he will join the X-Men sometime in the future, as seen by Xavier's "visions" in the same episode, it's unclear as to whether he's a member at the end of the series or not.
Personality: Colossus iz big. And he iz strong. And he iz Russian. He's a bit quiet, though, and kinda solemn, from what I can tell. He actually seems to be a bit of a caring person inside, but this was stifled by his forced allegiance with Magneto. The only time his "good" side really showed through is when he's with Shadowcat- he seems to have developed a soft spot for her. (Though Colossus has been confirmed by the creators as being in his late 'teens, even though he certainly doesn't LOOK that way...)
Final Reflections: Colossus was another character that really needed an episode that focused on him- most likely on his past, about how he got involved with Magneto, how his family got kidnapped, that sort of thing. We never really got to see a whole lot of him when he wasn't punching stuff, which was a shame; I especially would have liked to see more of his personality after he left Magneto's team for good early in the fourth season.