Bea Polley, of course.
The Polleys live next door—well, as close to “next door” as one can be to a huge estate—to the Deys. Nerissa grew up in Beverly Hills, but she spent her summers in England, and so she’s known Bea (short for Beatrice) since childhood. Even though the actual amount of time they’ve spent together is relatively small, they’re in constant contact. To Nerissa, Bea feels “real” in comparison to all the other girls at her boarding school.
Bea is of mixed race and has hair she can’t do anything with, which is why she prefers to style Nerissa’s. Bea has real talent in that area. Bea is also sane and acts as Nissa’s conscience and voice of reason.
In short, Bea is Nerissa’s Horatio.