Infinity has more characters than you can shake a stick at. A magical stick, of course.
Stone Blakemoor is a bully who starts a fight with Nick but doesn't get in trouble…because he is popular and Nick isn't. Nick is mad that Stone is talking trash about his mother, but, to be fair, Stone doesn't say anything untrue when he says that Nick's mom was stripping for his dad the night before. She is a stripper.
Mom's friend and Nick's godmother is a "a voodoo priestess and midwife" (4.10) named Menyara. That is an awesome combination of professions, but we don't ever actually get to see her do her job. Instead, she simply dishes out advice, such as this gem: "Aunt Menyara always said no one could make him feel inferior unless he allowed them to" (1.24).
At school, Nick meets Brynna Addams and her brother Tad, who are "decent" (1.170) and later turn out to be Dark-Hunters. Nick also has three "friends" named Tyree, Mike, and Adam. We put friends in quotes because these are the friends who mug a tourist couple and, when Nick tries to stop them, turn their gun on him. We never learn their fate, but getting thrown around by Kyrian has to hurt.
Kyrian's housekeeper, Rosa, is a trained fighter (surprise) who makes a mean gumbo. She's a better cook than Nick's mother, but Nick doesn't tell his mom that. Even though she seems to be half ninja, Acheron still wipes Rosa's mind after killing a bunch of zombies in her presence. What exactly did he do that she needs to be kept in the dark about? Maybe people just aren't really supposed to see him.
Other adults Nick meets include Liza Dunnigan, who tells Nick how to pronounce Acheron's name. "'With great respect.' She winked at him. 'It's Pahr-thin-oh-pay-us. Ack-uh-ron Pahr-thin-oh-pay-us'" (8.217-8.218). She doesn't elaborate on how "Ash" is a nickname if his first name is pronounced with a hard "k" sound, but we won't quibble.
Nick also meets Tabitha Devereaux, a Cajun demon-hunter who needs her stakes sharpened. Maybe she likes to cosplay as Buffy. And Caleb recruits his lawyer friend Virgil Ward, a "bloodsucking attorney" (13.53) who literally sucks blood. He's a vampire, if that humor went over your head.
We've saved the best for last. And by best, we mean worst. Adarian Malachai, "father and a monster" (4.63). You don't get much more blunt than that. Oh, wait, you can: "Absolute pure evil, Adarian Malachai was incapable of any kindness or mercy" (11.7). Adarian is Nick's villainous dad, a guy who is attempting to rebuild his army to take over the world or something. He doesn't want Nick hurt, but he doesn't want Nick to gain power, either.
Maybe he's more complicated than the pure evil he's made out to be, but for now, he's our resident monster—literally and metaphorically.