Bonnie and Beanie are two black twins who are basically interchangable. As Lone thinks, "they were like one split person" (1.26.29). He describes them as the "hands" of his unique life form, hands he can move "anywhere" (2.12.44). That's because they have the power of teleportation. Lucky.
Bonnie and Beanie play a minor role in the book, mostly just fetching or ferrying things for the gestalt, making the plot move along more easily. Being part of the gestalt, though, makes them main characters.
The fact that they get such minor roles in the gestalt, on the one hand, might raise questions about the novel's treatment of minorities. On the other hand, they were among the first significant black characters to appear in U.S. science fiction. Remember, this novel was published in 1953, before schools were even integrated. (See the theme on "Race" for more.)
Yes, they start off with something of the same loneliness as the other characters that join the gestalt. They always play alone, and are at first teased by a five-year-old Janie, even though they soon become friends. Furthermore, they're of an ethnicity that is already discriminated against. But they have each other and appear happy.
Two attributes of their personalities stand out most: Mischievousness and courage.
The twins love to use their teleportation power to confuse people. This aids the gestalt in stealing food or undertaking other tricky tasks. But sometimes the twins teleport just to tease people. For example, they tease Gerry by confusing him when they first meet. That's when Bonnie and Beanie teleport around in the shelter, disappearing and re-appearing, confusing Gerry all the more because there are two of them and they look exactly alike.
They cap off their pranks with their signature phrases: "Ho-ho" and "He-hee." Because that's not corny or anything.
At the climax of the novel, Gerry nearly manages to probe Hip's mind and defeat him once again. But Bonnie restrains him, allowing Hip to subdue the head of the gestalt. This shows that the twins are brave and good-natured. They probably want the gestalt to be the same.
The twins can teleport. Blip! One is described as disappearing "like a squirted appleseed" (3.16.119).
It's notable that they must be naked in order to teleport. At first, that might strike you as a very random requirement. But consider how it contrasts with Mr. Kew's philosophy. An abusive, twisted person who murders his own daughter, Evelyn, Mr. Kew condemns nudity and sex as evil. The good twins, on the other hand, must be naked to work their teleportation magic—something that the novel thus views as good and part of nature.