And here I was worried about whether I'd made a Greg that was an OC, based off of some of what one of the readers had said before.
The reader's relationship with Wolf Spider!Greg is rather different than canon!Greg, even if he's recognizably the same character.
In canon, we learn that he's at best tolerated by Taylor. He presumes that they should be friends or more because of their shared outcast status. But to paraphrase, the popular clique is all alike but the outcasts are all outcast in their own ways. It's not really a point of commonality and Taylor's pretty clear she wants nothing to do with him and he doesn't seem to care.
In Wolf Spider he's established to actually be her friend. And while he's awkward and sometimes does presume too much, he still does his best to support Taylor as a friend. We get that characterization from the start.
Wolf Spider's Greg isn't necessarily a different person at all, but Taylor's relationship to him is very different. That difference in relationship makes Greg's crush way less creepy, and through that difference he actually knows and treats Taylor as a person.
An aside…
Greg kind of gets automatic minus points from me (and I imagine others) due to him being uncannily like poorly behaving boys and young men I grew up around. It's easy to project them on to him. There are things that he didn't do in canon, but because of that context, it's easy to imagine him doing, and those get mixed in with his character.
Others, I suspect, relate to the nerdy outcast guy and then resent the other half of his characterization, someone with poor judgment, obsessive behavior, boundary issues and a constant need to build up him self up even with the most obvious of lies.
As a result, he's pretty widely disliked, fairly or not. So when his characterization here managed to actually make him sympathetic but flawed, that got a big reaction.