How do I level you up and increase your power? Do I just have to increase our friendship levels, or do you have to fight? Speaking of which, while relationships are complex and don't exist on just one axis, this is still... y'know, a video game, so there has to be a way of gauging our abstract progress to "maximum friendship levels?"

It's impolite to ask that question, peon.

The co-player is only attempting to get ahead of the game and attempt to do it in the most streamline way possible.

It's bad manners.

I guess. I will tell the co-player anyway! You can increase friendship in the same way you would increase friendship with a normal person, by not acting like a creep about it. In other words, just keep being yourself! Stop focusing on the fact that this is a video game, if you keep thinking to yourself it's a videogame you'll just keep losing reasons to keep this up! We can also gain levels by fighting. The best way to instantly gain a 100% friendship is to solve our "personal problems", but good luck with that co-player, I have no major flaws to solve!

You can just bring up a screen to view all your "relationships" and their levels, but I would advise against doing that too much, we all know when you're looking at it and countless bringing it up is frankly awkward. You should be able to find it if you look around.