I have to agree. The relationships between Ash and pretty much all the characters is spectacular. They are all complex in their own right. Even his relationships with antagonists have aspects of caring and sympathy to them. Seen in relationships like Ash and Dino, where though Dino can be the scum of the Earth, he also is protective and clearly cares for Ash in his own sick and twisted way.
The plot does shift focus pretty abruptly. This is true. I'll be pretty honest, the first time I read the ending I was mad, though, and pretty much refused to read anything else for a good few days. It made me mad that Ash died, especially the way he did, even if I saw it coming. At this point I think I've resolved the ending a bit with myself, though.
A lot of people have commented that Ash presented as a Gary-Stu, but I don't think that's entirely true. Ash may be gifted as an intellectual sniper fighter, but he has many deep personal flaws. I like how in a lot of ways, Yoshida held Ash accountable for his situation, particularly in the manga's end. Ash was a victim, yes, but Ash was also offered many outs by people who cared about him, especially Eiji, and yet he never took them. Ash isn't perfect by any means. He is self-pitying, self-destructive, and a risk taker with dodgy morals. Even if he ultimately moves beyond this mindset by the end of the manga (racing to the airport after reading Eiji's letter to save himself instead of wallowing in his pain), his past decisions and relationships still come back to bite him. Notably the person who killed him isn't someone of whom he was a victim, such as Dino, or Foxx, or Blanca, or Yut-Lung. It's an enemy he made from a choice he made, because his own flaws created as much of his problems as his circumstance, and those choices don't just go away. I think that's enough to land him out of Gary-Stu territory. He ain't no saint.
And so with that analysis I really ended up likely Banana Fish thematically: you can't escape the past. You're your own worst enemy. True that. For me a lot of the tragedy came from the fact that Ash was able to grow as a person, but he did so too late for help. Although I will concede the side stories were pretty amazing as well (albeit the t-shirts a bit distracting).
Love this manga. Definitely one of my top and probably the one I have most personally resonated with, even if it took me a while to get it. I have it as a 9/10 for the weakness in plot towards the end (even if it was more thematically strong), and the terrible art for the first few volumes. Seriously, some of her panels have people (not chibis) three heads tall and floor tiles with no perspective the sides of their heads...