I know the basic trainers can be spent on any character and they upgrade experience levels (Basil, Brady, Burt etc).
There are also the Persona Trainers like Peacekeeper Trainer Phebe and Citizen Trainer Caitlin of whom upgrades a characters RUSH if you use the corresponding Persona characters but you pretty much waste it if you use them on other characters without the same Persona.
What about Fast Trainers, Legend, tough trainers. How are they best to be used? I see the fast trainer Frances doesnt give a rush symbol to any of my fast characters so I have to use them on characters that have active skills and it will show an active symbol? Not sure I have any characters with Active skills atm haha.