Sus Harper

He can defo read minds but I want to know how and who he is

definitely agree!! there were little details appearing now and then, and the last chapter made it even more clear
I mean, even before this ch. there were several times when he suddenly started laughing(and it was right after Achilles had thought of smth funny)
however, it seems to me that either his mind-reading ability is a bit special (for ex. activating only when there are *strong* emotions/clear memories, because there are times when he doesn't react at all, unlike the other characters-mindreaders that I've seen) or he is really good at maintaining his facade(because sometimes he makes the other person reveal their inner feelings with asking some seemingly innocent questions??)
and also I've seen an interesting theory in the comment section that Harper can turn out to be Patroculus, hwvr, I personally don't think that that's the case...

Now that you said it, right... It must've the intensity of the emotions of a person that might trigger his mind reading.. oh wait this can be the reason he is a therapist? Like the intense emotions which leads his triggering which helps him helping the patients. Hm. And also like you said, I think he might be having this facade to make sure his cover doesn't blows up because he can't be a therapist without knowing how to control oneself? Just my take and as for Patroclus part, I don't think he is cuz if he was, then surely enough he would have memories retained with him which might have helped him easily with Achilles but he doesn't which clearly shows how he never even slipped up the real name.

I think there's some kind of precondition of strong emotions there. He doesn't respond to everything Adrian thinks, eg him not knowing that Adrian is attracted to him and being surprised at his admission of wanting to kiss him.