Touko and Shigeru Fujiwara- Natsume's aunt and uncle. They live nearby after wanting to stay in America for a whie just to see what it was like. Natsume complied because he didn't want to get in their way and attend PLC because of it.
Reiko Natsume- His grandmother. She died young but left Natsume with a very unique inheritance that's gotten him into loads of trouble.
Nyanko-sensei- Natsume's bodyguard. And friend, though they both refrain from admitting it on most occasions due to their frequent arguing. Natsume knows that Sensei will take care of them, and probably less because of the Book of Friends than that Nyanko's developed an earnest fascination and like of Natsume.
Kitsune- A small fox spirit that's very affectionate toward Natsume after Natsume saved him from some bullying by bigger spirits. He followed Natsume to PLC after a while and is now going to stay with him for a while.
Makoto Kino- Natsume likes Makoto. She's strong and protective and understanding and at the same time doesn't overload him with questions about things he doesn't desire to talk about. He feels fairly comfortable with her, though most of the time he won't share a lot of things with her because he knows that she can't see what he's talking about.
Usagi Tsukino- Natsume has a lot of trouble with Usagi. He likes her but can't handle her 9/10 times because of her imposing and meddling personality. She refuses to let him keep secrets and gets offended at a large amount of things Natsume says, whether he means it negatively or not and it makes Natsume have a difficult time conversing with her. Despite that Natsume feels extremely uncomfortable around her, he knows that most of her nosiness is routed in concern and that Usagi doesn't mean anything bad by it. He wants to try to get along with her still, but it's a very difficult friendship to maintain, in Natsume's opinion.
Yuzuriha Nekoi- Natsume was excited when they first met because they were both able to share something that they hadn't been able to before-- Natsume was the first person that Yuzuriha had met that could see Inuki and Yuzuriha was the first person Natsume had met around campus that could see... well, anything beyond what the average human being could. Though she's only really able to see dog spirits, Natsume and she still get along pretty well.
Peter Petrelli- The only person on campus who can actually see everything he does and Natsume appreciates it. Moreso than he probably would anyone else because Peter and he see eye-to-eye on many things, including the lack of need to choose between humans and spirits. Unlike most of the people Natsume has met who can share the same scenery with him, Peter doesn't have anything against the spirits, though Natsume theorizes that's because Peter had never seen them until he had run into Natsume the first time they met and didn't have time to grow up with the same situation Natsume did. Whatever the case is, Peter's existance on campus is a source of great relief to Natsume and Peter is one of the few people Natsume would openly acknowledge as a friend, despite certain circumstances surrounding mass memory loss around the school.
Rei Hino- They just met. He's pretty sure she thinks he's weird.
Hatsune Miku- Just met. He loaned her a textbook during class.
Kafuka Fuura- A strange girl by all accounts, she actually manages to make Natsume feel fairly normal. He likes her for this, but they don't talk enough to really be friends yet. She did get him to join drama club somehow, though.
The Kappa in the Pond- One of the few spirits that actually stays at PLC. Natsume talks to the kappa and directs him back to the pond whenever he ends up wandering away and getting dehydrated somewhere on campus.
Susuwatari- Soot spirits that reside in the school. He helps them whenever they need it and they do the same for him, even if they're extreme curiosity makes them a little troublesome after they got over being shy around him.
Hinoe- Female spirit who hates males but has allowed Natsume to keep her name. Natsume and she get along fairly well for her hating males, though probably due to his grandmother's influence
Misuzu- A very large, horse-like spirit who told Natsume to hold onto his name because he didn't dislike being called by him. They're not exactly friends, but Natsume does trust Misuzu to protect him if he needs him. Misuzu has said more than once that he would gladly replace Nyanko-sensei as Natsume's bodyguard.
Various other spirits- Because of Natsume's ability to see spirits, he runs into a number of spirits that only Peter can really see. Whether or not he befriends them... well, that depends. Most of them just want their names back or some kind of aid, but some of them figure that killing him for the Book of Friends seems like a good idea.