Awesome! Thank you so much for this, it helped a lot.
Anyway, I think I already found something pretty interesting. At first, I was just trying to edit my rental pass trainer's skin tone, but I accidentally stumbled across something neater than that. By editing 0x23895 (where my first/main rental pass is) from 06 to 07, I was able to change my trainer to Lucas from DP. His trainer class is considered "Young Boy A" in the game, although he has his own unique animations. Weirdly enough, his Battle Pass portrait is messed up; it zooms in on his legs rather than his face or full body!
I'll try more values to see if Dawn, the boss trainers, or alternate skin tones are available.
Edit: I have just tested out values 08, 09, 0A, 0B, and 0D. 08 let me play as Dawn, who is also called "Young Boy A". 09, 0A, 0B and 0D crashed the game upon loading Battle Passes.
Basically: 07 = Lucas, 08 = Dawn. 06 and under are all legal trainer classes.
I've also found out where the skin tone is determined. It is at offset 0x23A62. The default (A) skin tone is 0, but setting it to 1 will change it to B, the slightly darker one. Setting it to 2 will likely make it C. It was probably located here since the Japanese version didn't let you change skin tones, so they placed the setting in a near-inaccessible location.
I've also been trying to search for where rental Pokemon are located, but haven't had any luck so far. Any help would be greatly appreciated!