So if you had the Mike x Stella scene you have done everything possible on the Stella-route for now. She should come to the gym on her own once these two events are completed:
- You must have learned the second special move from Jade (Ginpsey Roll) requiring Skill Lvl 4
- You must have seen the second training event between Aline and Jade (Train together with Aline, do "exploration" in town until the event happens)
From there the story contiues with her and Jade more or less on it's own. Watch a few scenes with Stella happening and then the final "training together" event of the update becomes available.