You mean the RE6 characters that Capcom & Valve agreed to put in L4D2?
What is never ok to developers is stealing their models and then using them. But what can oft get the go ahead is fan made projects using an IP. You’d get more out of having someone create a model with clothing like Ellie and then distributing that for non-commercial development. Of course some projects such as Main5’s Fighting is Magic an homage to My Little Pony: Friendship is Magic which had its development ceased after Hasbro sent legal notice.