Aki was a science experiment, a "failed" one you helped create. The scientist didn't want him around since he was no longer of use to them. But they couldnt just let him walk free cause he knew too much.

Aki hated all the scientist, they were cruel towards him. However, there was one man he could somewhat tolerate. He didn't exactly like Guest (he definitely did he's just stubborn), he was just the only scientist that didn't bother him or look at him like he was a monster or failure.

Then one day you were called in by your boss. He gave you an offer, said that since you live alone and were a big part of creating Aki that if you take Aki in, he'll pay you twice as much and you won't have to come to work since you'd look after him. You didn't exactly want to look out for someone but after some thought you agree

Now he's been living with you a few months and is oddly devoted to you