Stella not only suffers from depression, but also from a neglectful father and a clinging mother. The parents use Stella as a weapon to launch at each other -- each accusing the other of being the worse parent. Traumatized by her parents' behavior, Stella seeks comfort where she can and stumbles across moody intellectual Kevin at a party. When Kevin isn't drowning himself in existential literature or contemplating doom and gloom, he practices self-harm -- a behavior that he too weaponizes against Stella, but claiming that she bears some responsibility for it. Barely into the relationship, Kevin's nasty side appears as he launches into abusive jealous tirades against Stella's friendships with other boys. Stella, desperate to keep the one thing she feels is "safe" puts up with Kevin's abuse. You get the picture. It all ends (at the start of the novel) when Stella accepts that she can't help Kevin and needs to just let him go.
Like Sarah Dessen's Dreamland, it is pretty obvious from the beginning that Kevin is manipulative and Stella is manipulatable. Between her depression and low self-esteem and the poor parental exemplars, she's completely unequipped to deal with Kevin's head games. Any sane person would run far far away from him, but Stella is easily sucked in.
The light in this otherwise grueling story, is the authenticity of the characters and the insights they shed on themselves. Stella, her mother, and their friends all seem enlightened (not that it halts the corrosiveness of their relationships). The males are (Kevin and Stella's Dad), however, are pretty useless. Whether those insights would actually make a difference seems unlikely given that the type of people who fall into these toxic relationships generally don't accept advice from others.