Can someone explain to me the story line? i don’t really get it. who’s hachi? is that ...

There are tow hachi. One hachi is Himemiya's dog that he had as a child, the dog later died after being hit by a truck while he was running after Himemiya. The other Hachi is the delinquent currently lovers with Himemiya. His full name is Hachiko. Himemiya got drunk (when he drinks he gets crazy and that'swwhy Mao, the leader of his gang, banned him from drinking) and mistook Hachiko for his dog Hachi because of Hachiko's countenance. They had sex but that wasn't merely because he thought Hachicko was hachi, he was already attracted to Hachiko at first sight (Himemiya later questioned himself too as to why he had sex with Hachiko if he merely mistook him for his dog) . That explains the sexual part of his drunkenness. Hachiko started acting like a dog so as to get Himemiya to love him cos he thought Himemiya's type is a dog in the literal sense of it (this is cos Himemiya kept insisting that he only had sex with Hachiko because he mistook him for his dog Hachi) . But he later reverted back to being his normal self (though panicked and desperate) when he found out Himemiya may love someone else (mao). But after all his efforts to be with Himemiya , he is now lovers with him. I hope this answers your question

YES! this is perfect! haha i understand now thank you! <3