When Warren Worthington (Angel) is introduced, he has no connection to Xavier or the X-Men. It's even demonstrated that Cyclops and Professor X don't know who he is. In "Proteus: Part I" and other episodes throughout the series, flashbacks showing the first X-Men include one with big angelic wings on his shoulders. This same winged X-Man shows up in "Xavier Remembers" in a way that confirms he is Angel. Specifically, Xavier has a dream sequence where he sees the X-Men of past and present wearing their original costumes, then sees them transform into their current costumes. One of them is original X-Men uniform Angel, who transforms into Archangel.