A very large man made of metal who keeps trying to recruit Deadpool into being a better person, entirely sincerely.
Colossus exists in the Deadpool films to be sincere at somebody who cannot handle sincerity. He is patient, Russian, enormous, and genuinely trying to make Wade Wilson join the X-Men, and the joke works because he never gives up and never gets angry.
Three actors have played him across the run, which is easy to miss because he is computer generated from Deadpool onwards. Stefan Kapičić is the voice most people mean.