*Spoilers. If you have not read these books, beware. Spoilers*
Sebastian is Clary's brother. And he is pure evil. Like there is way more evil in him than good. Unlike the other two boys I have highlighted, I am not for Sebastian at all, but there are tiny tiny moments where I feel like he got the short end of the stick. That he is capable of caring. That maybe his love for his sister gives him a bit of creditability.
“What do you think it would have been like if Valentine had brought you up along with me? Would you have loved me?" Clary was very glad she had put her cup down, because if she hadn't, she would have dropped it. Sebastian was looking at her not with any shyness or the sort of natural awkwardness that might be attendant on such a bizarre question, but as if she were a curious, foreign life-form. "Well," she said. "You're my brother. I would have loved you. I would have...had to.
Now the reason I highlight Sebastian is because I find him to be truly on of the more evil characters out there and I love how Clare really gives insight into him, that he has the ability to seriously cause damage. Plus the brother/sister relationship makes you wonder how well they represent good and evil.
“Why did you?” Clary asked. “Why did I what?” “Help me back there.” “You’re my sister.” She swallowed. In the morning light, Sebastian’s face had some color in it. There were faint burns along his neck where demon ichor had splashed him. “You never cared that I was your sister before.” “Didn’t I?” His black eyes flicked up and down her. “Our father’s dead,” he said. “There are no other relatives. You and I, we are the last. The last of the Morgensterns. You are the only one left whose blood runs in my veins, too. You are my last chance.”