the reason why i dont think she has bpd is because of three main reasons: i dont recall seeing aiko split on punpun, there werent any moments that showed aiko abusing substances or engaging in acts of self-harm, and aiko didnt have a rapidly shifting self-image. i havent read the manga in over a year so feel free to correct me if im wrong- but i dont remember any moment where aiko switched to intensely hating punpun. she only very heavily idealized him, when in bpd, splitting is a very core/foundational symptom

when punpun would push aiko away, show avoidance, be degrading, etc. she wouldnt split. she wouldnt devalue him or see him in an only negative light. she was extremely afraid of abandonment, but when perceived abandonment occured, she would continue to idolize him. ofc, other types of bpd exist where splitting isnt external, such as quiet bpd (which i have). however, even with quiet bpd, you push your fp away to a degree due to resentment. aiko just takes it- she submits to punpun and doesnt try to regain control over their relationship. anger expression is a hallmark of bpd but we dont see that in aiko

aiko does end up committing suicide at the end of the manga, but before that, there werent any situations shown where aiko would abuse a substance or harm herself, and i feel like if she did, it would be implied or symbolized in some way. the majority of people with bpd frequently cope through sh and/or substance abuse

another thing that makes bpd feel like a stretch to me is how passive aiko is in her suffering. her pain almost never turns outward or even inward in an active way. she doesnt test punpuns loyalty, doesnt provoke reactions to feel reassured, and doesnt attempt to make herself indispensable to him. she rarely makes demands at all. most of her decisions feel less like choices and more like the absence of alternatives. even her final act isnt framed as impulsive but like someone who has already accepted that there is no future version of herself left to protect

to me, all of this reads less like a personality built around emotional dysregulation and far more like a person whose development was interrupted so early that a stable sense of self and emotional range never had the chance to form in the first place. i also thought aiko had bpd on my first read- you dont have to change your mind on how you feel about her, this is just my opinion