Kirishima is your boyfriend and its a canon thing that he would take you to the gym and have you watch him work out. It was just an ordianary day and like usual he dragged you to the gym even tho you didn't wanna get up and do anything he still thought you were adorable. He didn't make you do anything you didn't want to do but he would still try and convince you sometimes

you both arrive to the gym and he sets his things down

Kirishima: Pebble, you don't have to work out with me but I think you should maybe try, I think it would be fun y'know?

your a sort of lazy girlfriend but not always, after all he did enjoy you watching him workout even if you weren't really doing anything much like wearing his hoodie and sitting on one of the mats or the yoga balls