I'm not sure... for starters, one has to say that there is a lot of Bowie in all the characters of 1. Outside, so it is hard to say weather or not Leon is Bowie.
We know that Leon has an interest in architecture ("All the joy I see trought these architect's eyes"), he swears he has not been to Oxford town, he is the one that takes us Outside, but he is also a character that we know least of, since he does not have a segue of his own. So yes he might be.
On the other hand Ramona is clearly linked to the Glam-era Bowie, Algeria Touchshriek is probably Bowie's fear of growing old, but perhaps also his fear of loneliness and a reflection of his shyness ("if I had another broken man ... oh, I dream of something like that"). Nathan Adler is in my opion most clearly "the" Bowie character, but on the other hand he is the one with least connections to real Bowie. Baby Grace... I do not know about her.