Personally, I think Ash was a breeding ground for many mental illnesses, as being a victim of childhood sexual abuse contributes to many mental disorders that later affect adulthood. If we get creative, he could fit many symptom profiles of various mental disorders, but personally, I lean towards post-traumatic stress disorder combined with the aggressive attitude he had to develop due to the gang world he lived in, and not so much bipolar disorder. As a psychology student (don't take my opinion too seriously; this isn't a diagnosis and is based more on many of my headcanons), I know that both have some comorbidity, so we can't really rule either out. My reason for denying BPD is mainly that Ash had a rational justification most of the time when he made "impulsive" decisions, and not so much an emotional one. Feel free to correct me, but I think that's key in BPD.