At his heart, Peter Rasputin is a fairly simple man. He is incredibly loyal to his family, willing to forgive his brother Mikhail for forcefully trying to take over the Morlocks or even going to painstaking lengths he went to try to find a cure for the Legacy Virus that was killing his sister Ilyanna. He extends this to his friends, whether it be standing up for Nightcrawler and Wolverine in a bar fight or even to his on-again, off-again girlfriend Kitty Pryde.
Sometimes his selflessness is a detriment, as he sacrificed himself to find a cure to the Legacy Virus which lead to his demise. But this is comics so he was eventually reincarnated. He sacrificed himself to become the Juggernaut in an attempt to save San Francisco.
Colossus is a great supporting character because he is so generous. He is extremely likable and just a nice guy. You can just tell that he would much rather be home in Russia farming and being with his family instead of being a super hero. That makes him cool in my book.