Amanda Monk is Violet's friend from her cheerleading days. Since Eleanor's death, they've grown apart. Violet's not into cheering these days, and Amanda is cheery as ever—or so it seems. Toward the end of the book, we find out Amanda's cheerfulness is nothing but an act. She tells her suicide support group:
"I am not happy at all. My mother is making me come here." (44.15)
It turns out she has bulimia…and has tried to kill herself twice. While Amanda is terrified her secret will repulse people, it ends up bringing her and Violet closer together.
Ryan Cross is the Bartlett High hunk whom Violet was dating when her sister was killed. They're "on a break," but Violet's pretty much donezo. He's the perfect guy (well, assuming you don't mind kleptomania). Ryan mostly seems like a cardboard jock, but Finch's descriptions of him are probably the most perfect in the book:
His hand [is] on the back of Violet's chair as if he's the host at the Macaroni Grill. Guys like Ryan Cross have a way of reminding you who you are, even when you don't want to remember. (10.28)
He has nothing to worry about, though; Violet's Team Finch all the way.
Gabe "Roamer" Romero is the bigoted bully figure at Barlett High. He's the kind of charmer who uses gay slurs and punches you in the face for making a harmless "your mama" joke. What's interesting about Roamer is that he and Finch used to be besties:
A few years ago, I asked my then good friend Gabe Romero if he could feel sound and see headaches. The next day it was all over school, and I was officially Theodore Freak. (18.18)
Sounds like Roamer has been the author of a lot of Finch's misery, at least at school.