Unfortunately, there are no other characters in the game that could be developed similarly to the trainer. The other significant characters are men. Although maybe I'll come up with something in the future.
[Spoiler!] If you read 2 books on sports, you can do strength exercises with a trainer. If you progress in this, Astarte will offer you candies that increase strength. The trainer, noticing your unnatural progress, will demand an explanation. Then you can offer her candies.
