Mr Spencer is old and decrepit. Holden relates that he is so stooped that whenever he drops the chalk at the blackboard, one of the students has to pick it up and hand it to him. Holden thinks this is a real affliction. He genuinely pities Mr Spencer on account of it. However, when he visits him to say goodbye after being leaving Pencey, he regrets it. Mr Spencer is sick and sitting around in his bathroom and Holden can hardly stand looking at him. Not only that, he is unpleasant and sarcastic towards Holden over his lack of academic achievement and lectures him about growing up. Yet at the same time he appears genuinely concerned about Holden's future. He is therefore presented in rather an ambivalent light.
Selma Thurmer is the daughter of the headmaster at Pencey. Holden describes her as follows:
...she wasn't exactly the type of girl that drove you mad with desire. She was a pretty nice girl, though. (chapter 1)
In other words, Selma is a likeable girl, although not very attractive. Holden goes on to explain why he likes her: because she doesn't boast about her father. In fact, Holden appears to think that she shares his own low opinion of Mr Thurmer.
Allie, Holden's younger brother, died of leukemia and Holden misses him terribly. He frequently remembers him and even talks to him. Holden remembers him as a vey intelligent and nice boy, with a great sense of humour. To Holden, he represents the ideal companion, and also functions as a symbol of lost content. When Allie was still alive, Holden was still really a child. He tells us he was only thirteen when Allie died. Allie, therefore, died just as Holden was entering adolescence, which is never an easy time. Allie's death has clearly made things even harder for the adolescent Holden.