1. It makes your butt bigger. You can see the items, their effects, and their ingredient requirements over here on the wiki: https://summoned-by-accident.fandom.com/wiki/Player%27s_House#Body_Potions_.28Kru.29
2. By teach, you mean have him level you up. If you picked someone else to level you up the first time, then you can't change afterward. You have to use the same level up trainer permanently.
And if you haven't leveled up at all yet, then the requirements for leveling up to 1 is to have encountered the Drunk Wolf fight before and then level up with a trainer. Then leveling up to level 2 requires you to win against the Street Horse or Street Rat 3 times. And then you'll have to use the same level trainer you did before.
A lot of the paths in the game are going to be mutually exclusive and so it's good to keep saves before all the important choices and events, so you can easily go back and pick someone else or do a different event path. Such as how the companion you pick for Main Story Quest 1 will determine what job is available to you later. Though the only one available right now is Kuri's job, which requires you to take Kevin to the quest with you.