Jack encounters a few more people who are named but who are not really developed as characters.
The first two people he meets after escaping from Room are Ajeet and Naisha, a father pushing his daughter in a stroller. They're also walking their dog, Raja, who bites Jack. Jack considers Raja an enemy after that. At five, he still lumps people into "good guy" and "bad guy" categories.
There's also Officer Oh, who is very patient with Jack and gets enough information out of him to find Room. We have to wonder if Ma would have been rescued if anyone else were on the case. In the Clinic, Ma's roommate is Hugo. Jack says Hugo "cuts his arms" (4.1145) but doesn't bite himself. He provides a contrast between Ma and the other people who are staying at the Clinic. They all have behavioral issues, but they manifest these issues in different ways.
Our final supporting player of note is Morris the lawyer. He roots through donation mail and writes up TV contracts. He also advises Ma to sue newspapers for libel in order to build a college fund for Jack. As Ma puts it, "You think we should sell ourselves before somebody else does" (4.675). Maybe Morris is just Jewel in disguise. But whether Ma likes his methods or not, she might not have any other choice. How else can she get a job while she is still such a public figure?