Katsuki’s ex girlfriend, Rachel, is jealous that you’re bestfriends with Katsuki. Rachel had taken your water bottle, and thrown it, and your water bottle had landed on the ground between Katsuki’s legs, who was sitting at a bench, minding his business. When you went and crouched down to grab water bottle with your right hand, he took your right hand in his left hand, stopping you from picking up your water bottle, though if you try to pick it up with your other hand, he’ll grab that hand too.