Responses
There's an interesting excerpt from Liar Game (of A), that goes "to doubt is to trust". Because sometimes trusting unconditionally is on the same coin as being indifferent. Ultimately, I feel that's what Phos originally tried to do. Because Adamant has well...adamantly run away from his responsibilities. Whether it be an explanation, a resolve, or a solution. It hurts that Phos lost so much of his innocence in the pursuit of the truth though.
I don't think Cinnabar nor Phos are wrong for fighting for what they think is right, but Phos is filled with malice and hate that it hurts to see them fight someone who they genuinely wanted to help in the beginning.
Cinnabar is Phos's best foil in that they could have made the same choices, but instead of doubting Adamantium and seeking the truth, they decided to trust him until evidence showed otherwise. And to be fair, no evidence showed he was evil, Phos just wants to help the Lunarians and get revenge (mostly the latter). At least, that's what I think.