So I was re-reading the manga today and actually Ein talks about his name with the police officer (after Bexan and Diesel's fight). He's Korean, his surname is Lee and Ein is Germanized version of his Korean name or something: http://www.mangago.zone/read-manga/wolf_in_the_house/nmt/c015/12/ And maybe werewolves don't have surname so Diesel it's just Diesel.
Well spotted, however "Lee-ssi" would still make "Mr Ein" as you don't just say the surname without a title. Diesel might know him as Mr Ein, rather than his Korean name, but it seems odd that he doesn't call him by his first name. Diesel is a surname, and this too would carry a title, and it seems unusual for either wolf to only be known by their surnames for reasons already addressed. Japan seems one of the few places where calling someone just by their surname is acceptable and denotes respect, rather than it coming off as strange, rude or impersonal for a German setting.
I think that Ein is his first name (whatever exactly is that in Korean), maybe his Korean name sounds a bit like Ein and he decided to use more Germanic version when he was abroad (although I admit it sounds super weird to be called One). It's clearly stated that his surname is Lee and he is referred as Mr. Lee a few times by Minsuk. Later on Bexan and Minsuk start to call him Ein.
And Diesel is a surname for us, humans, but he is a werewolf/dog and you can totally name your dog just Diesel. As far the story goes no one of the werewolves has a surname, so I think it's wrong to assume that Diesel is his surname and his first name is a mystery.
Even as a word it is nonsense! Again .. IF YOU ARE NOT GERMAN, YOU DO NOT UNDERSTAND IT!!
I'm just gonna assume his Korean name is Ah In (아인) cause it's pronounced the same way as Ein.
How do you get the name "Ein" ?! I'm German and that's a number, a number that you do not take as a name ... that's like calling "Come on `one`"
For Koreans it must sound great but it is nonsense. ┑(￣Д ￣)┍ ..Sorry.
But the love scenes are beautiful and very intense