Ringer is by far the competent person on Ben's squad. She's dubbed Ringer because she helped the squad rocket to the top of the base-wide competition. (Ringer's a nickname, in case you're wondering. We don't know her real name.)
She's sixteen-ish, but she has a lot of gravity for her age—we're guessing she was a pretty high-ranking member of student government, back in the day. She never smiles—she tells Ben "it was the first thing to go," (57.4)—and she's razor-sharp. Her love of chess gives her an analytical outlook on life (or it may be the other way around).
In any case, she's scary-smart. We wouldn't want to meet Ringer in a dark alley, but we really wouldn't want to go up against Ringer in a game of Scrabble or Boggle.
Though we don't know that much about her, Ringer's important because she's the person who figures out that Camp Haven is secretly run by aliens. That's massively impressive: everyone at Camp Haven is indoctrinated to believe that they're fighting aliens. But Ringer's skeptical. She asks Ben:
"Does it matter that they got everything up and running after the EMP attack? That they're operating right underneath the mothership, gathering up survivors…? … Tell me the odds are insignificant that they aren't really them. Tell me what possibility I should bet my life on." (59.36)
Suffice it to say Ben would have never figured this out on his own. He can barely figure it out when she carefully explains it to him—we're not blaming Ben (it's mind-boggling info), but we are saying that Ringer's IQ is probably crazy-high.
Ringer has a tough exterior—she has to, in a squad full of guys—and she's circumspect; she doesn't say much. Still, she always offers a comforting word when it's needed most. After Oompa (a.k.a. Kenny) dies in combat, Ringer tells Ben that it wasn't his fault. Her words cut through his self-doubt and despair, which helps him stay on task.
She probably should have made sergeant instead of him. Just sayin'.