What was their relationship to each other like?

Is is mainly just mentor and student? Possibly some father and son dynamic?

I find it fascinating how Il-nam took In-ho under his wing and essentially left him this huge operation along with a large sum of money to have after he died. This shows a huge amount of trust between the two. In-ho happened to also be the one who closed Il-ams eyes out of respect after he passed. He was there in the building with him waiting for Gi-hun to leave, he might've helped taken care of him sometimes too. They probably spent a lot of time outside of the games near each other.

We might get more of them in season 3, but the dynamic between these two is one I want to learn about the most because there's so much mystery surrounding it. Everything that In-ho is today would have not been possible without Il-nams influence in his life. Il-nam changed everything for In-ho and In-ho seems to harbor deep respect for him while also embracing Ilnams convictions about the world.

I just keep picturing 2015 In-ho in complete despair after everything that happened with the games and with wife, contemplating suicide and refusing to touch the money he earned until he gets a card that tells him to go to a certain building. He goes there, he meets Il-nam, learns the truth about the games and starts accepting the things Illnam says about the world and the people within it to be true. Maybe Il-lnam offers him a role in the games, maybe In-ho asks if he could be involved. Either way In-ho leaves that room with a renewed sense of purpose and a changed man.