Of the other peculiar children, we see the most of Millard Nullings, which is ironic since he invisible. (Note the null in his last name—null means zero, or doesn't exist.) He is working on the "first complete account of one day in the life of a town" (7.80), by cataloguing every event on 3 September 1940. He's been working on it for twenty-seven years, a luxury he's afforded by his invisibility, which enables him to observe without being seen. Millard shows his bravery at the end trying to rescue Miss Peregrine, but gets injured by Golan's gunshot. Don't worry, they manage to bind his invisible wound.
Along the way, Jacob also meets Hugh, who can spit bees and probably grew up to be The Pain in Metal Gear Solid 3; Claire Densmore who "has a backmouth" (6.174); Olive, who can levitate; Fiona, who can grow flowers, and whom they call "Jill of the Jungle" (7.52); Horace, who dresses fancy and has prophetic dreams; and Victor, who is dead. All of the kids, aside from Victor, get one scene in which they assist the group, but other than that, we don't get to know them that well.
Not everyone has a superpower in this world. Jacob's relationship with his best friend (well, his only friend) Ricky lasts less time than Kim Kardashian's first marriage because Jacob can see monsters and he can't. At the time, Jacob thinks Ricky is lying and gets mad at him. Since Jacob describes their friendship as "equal parts irritation and cooperation" (1.1), though, he doesn't seem too broken up by it.
Jacob continues failing to make friends on Cairnholm Island when he meets Dylan and Worm, two rapping teens who play a prank on Jacob. He doesn't seem to be missing out there. Later in the book, the villagers blame the teens for killing a sheep, and even lock Worm up, but that's the last we hear about them.
Other characters who have a small bearing on the plot include Shelley, the Smart Aid manager; Aunt Susie, Dad's sister, who gives Jacob the Emerson book; Kev, the bartender who fries everything (we like him); Martin Pagett, the museum curator who tells Jacob about Cairnholm Man and is later murdered; and Martin's Uncle Oggie, who tells Jacob that the house was bombed on September 3, 1940, the day mentioned in grandfather's final words.