He was a manager assigned to him by Nympha, his company. It's implied that he was basically the first ever true friend he had and the only one who could handle him, but for some reason he ended up leaving the professor. Everything after this is a spoiler so don't read if you don't want to.
I'm caught up on the raws and there's still not much information about their relationship, the only things we know about his personality is that he was warm like Garam. Eva remarks upon this, saying that despite the professor getting warmer from Garam's influence, Garam is getting colder (though ngl most times he's actually justified so I disagree with her and think he's just maturing), just as in the past the manager also started getting colder by interacting with the professor too much, so that could be part of the reason.
The professor remembers clearly their time together, but cannot at all remember his face and he goes out of his way to ask Talinski for pictures of him, but there's none. It's also known that the manager died in a supposed accidental fire. He keeps haunting the narrative up to chapter 201
I tried to get as many free chapters as possible combining both lezhin Korea (1 episode free every 2 days through the daily log in event) and comico.jp which gives 2 free chapters a day (24 hour countdown which is story exclusive + 1 free ticket per day). After that I just bought 166 coins from lezhin Korea which was enough to finish all of the episodes and have some leftovers for following chapters too.
Little life hack I used was also opening up the raws on my laptop and reading them by using Google translate on my phone, so I would point the camera on my laptop screen while I was scrolling so that it could translate it quickly and I could scroll past, rarely the translation didn't make sense but I'm pretty sure I managed to understand good part of the dialogue and story

Can someone explain to me who that man was?? He used to be his manager then he left him... And later he passed away?? Anything more Abt their relationship?