See, that's just the thing. The people you describe (I hesitate to use the term "normal" because of the association between "normal" and "acceptable," but I would call them "typical," at least widespread enough that we all personally have known one or two of them), for all their faults, have still accomplished that extra, elusive feat: they managed to actually get girlfriends. Obviously those relationships never lasted, but they still happened. In contrast, all of Russell Greer's relationships have been mental constructs, phantoms, illusions. He has never had a girlfriend to dump him and for him to develop a subsequent obsession with. He has never, and will never, get to that point.
That's what's alarming about his behavior. Erika is probably one of only a few of the objects of his affection- because they are objects, in his eyes- to even learn of his existence. Obsessing over an ex-girlfriend is unhealthy at best and creepy at worst, but when you consider the social history- the promises made, bonds formed, and all of that- behind that ruined relationship, it is far more understandable than Greer's monomania regarding complete strangers.