Ya know...when I rewatched the show/ re read the manga... I honestly felt like Hachi was the one using Takumi, even more so than he was using her. I think he fell long before she did , where as while he is her soulmate and she does love him, I don't think she ever fell for him. That is to say, I don't think she went gaga for him (for lack of a better word). Meanwhile he clearly fell for her. This is why he tried /tries so hard to dominate her at times and cheats and plays hot and cold. It's almost like his way of trying to keep the upper hand, which he has had in all of his relationships , even over Hachi at one point....but clearly, as things stand now....that didn't last
Despite everything, I still can’t imagine Hachi with anyone but Takumi. He’s scummy, I know, and she deserves someone who doesn’t cheat, I know; but I can’t shake the fact that he’s really fiercely in love with her— even if it’s in more of a possessive, obsessive manner. And that frightens me for my future relationships— what if it’s no good, and everyone can see if except for me?