Another who/whom question

Does it matter? It's up to the teenager to decide.

I agree. I'd write it however your narrator would say it, regardless of what was correct according to the grammar books. Also, I very rarely hear the word "whom" anymore. I think it is a dying word. I hope Chase doesn't see this.

Agree with Bufty and the Bat.

But since you asked about the grammar, whom is correct: it functions as the object of can trust. It sounds odd because so few people use the word "whom" anymore.

It's 'whom', but I've never met a teenager, girl or otherwise, that would think to say so.

Myself excluded, but I don't know that I ever technically met myself.

"Whom" is correct. FWIW, I would have said it as a teenager -- I'm actually more flexible now. But it depends on your teenager.

To see why, think about rearranging and using a different pronoun:

"I know I can trust him."
"I know I can trust her."

p.s. -- Speaking personally, I love me my object pronouns, and they will pry "whom" out of my cold, dead . . .

You could also phrase it as a question, which to me, as reader, would be a trifle more dramatic:

Who can I trust?

caw

Or rephrase her: I don't know who to trust.

Or I feel so alone and scared. If only I had someone I could trust.