Okay, so this was a book, possibly part of a series, that I started reading as a kid, but lost the book before I could finish it. It is set in more or less modern times, on earth. I want to say at least the first part takes place in a city in either Mexico or Arizona, but I'm not at all sure. One of the main characters is a young man (I wanna say early twenties) who has either an older relative or mentor. Mentor is at least implied, if not outright said, to be VERY old. ETA: Young man is either still training in, or recently finished training, to use supernatural powers/magic. The setting is definitely fantasy, not scifi.
There is a monster that attacks the main character in broad daylight in the city. I don't recall many details about the monster except I think normal people couldn't see them? Or he/his mentor could wipe people's memories after. At some point the young man meets a young woman, things get somewhat sexual, so this was not a kids story at all (probably why it stuck with me all this time). Older man doesn't trust the girl. Young man has either acquaintances or siblings who also have powers, some of whom are either outright evil or just don't get along with the main character.
ETA: I just remember that, aside from the (not graphic) sexual content, thinking this was a rather dark story. Lots of interpersonal conflict aside from the man vs monsters fighting. Of course that is to say "dark" by the standards of someone in their early teens/late preteens.
Anyways, I understand this is a long shot, given the time frame and the gobbledygook I dredged out of my memory, but thanks in advance for anyone taking a shot at this.