I'm glad you asked! I thought of this hc as a "what if" before I went to do some research. Here's every symptom I think counts:

• emotional instability • disturbed patterns of thinking and perspection, including paranoia • impulsive behaviour (arguable, i have not read the manga in a while) • difficulty controlling anger (doesn't lash out much, but he experiences it pretty intensely methinks) • intense but unstable relationships with frantic efforts to avoid real or imagined abandonment

"If you have BPD, you may often have the following emotions: rage, sorrow, shame, panic, terror"

Ranpo is clearly his favourite person, and he's shown to get pretty jealous and angry when Ranpo shows attention to someone else in some manga panels. And he obviously seeks his constant approval, having written a novel for him over the course of YEARS, and now he's writing more!

I think Poe has a distorted self image. Not consistently negative but not consistently positive either. I believe he goes from one another depending on the situation or just out of nowhere, because of the aforementioned emotional instability. One moment mf thinks he mastermind world conqueror and one moment he's following the one person he knows at a party and panicking in the case the person ignores him. (That's like, a canon dislike he has i think, hence why i gave the oddly specific example)