Is he replacing a character already there, or are you trying to activate a new slot? I only had to use the unlock all characters cheat once, and after I had all 21 slots filled, I haven't had a problem since. I never had to unlock Psylocke, Avalanche, Polaris, or Shadowcat, since all 21 slots were already filled (with Sabretooth and Pyro, plus the 21st from Emma). They are all there in my menu without having to do the cheat again. In fact, once you edit the herostat, you can still load your old game with the new changes. I replaced Professor X with Psylocke, and she worked fine. I'm about halfway through my game, and I wasn't about to start over. *shrugs* The power values are the same as the character you replaced, so you still have to watch out for that. Psylocke has Leadership because I assigned that to Prof, so she's stuck with it. Other than that, everything went off without a hitch.