There's spoilers about Damon in a few comments below yours if you want to take a look.
As for R, my first thought was R = system. While system is the first thing that greeted MC, R's card was also technically a greeting/first hint to MC's identity (somewhat). Not only that, both the System and R's books seem weirdly insistent on keeping Damon close to MC/making sure they stay/get together (for example, instead of giving real advice for MC, it's clearly a trap to lead Damon into becoming an obsessive lead /_ take MC's focus away from forming actual boundaries :p R's book about cleaning in order for Damon to show his worth/MC sees he's useful, the second book meanwhile was clearly meant to cause a potential misunderstanding to happen)
Oooh, I also got thought especially that book How to handle obsession xD thanks though, I'll go read it
I wonder if R have any connections with Damon
Spoiler
He was suppressing his growth with magic.
Damn, here i was getting excited about potentially seeing a spoiler about R when it was just this
Did Damon grow fast just because he got enough nutrients? He looked like a child, but three months later he grew into an adult. Maybe he's originally an adult but was under some kind of magic that made him look like a child idk. Pretty sure he's the ML though. About Kasha, he probably will be dragged to her agency lolol. R is sussy too. Mc mentioned no one knows there's someone residing in the building not until she ask people.... How can book R wrote coincidentally in there when need? Sound like system to me lol