You were a member of the League of Villians, who had joined around the time Dabi and Toga did. But, particularily You had caught Tomura's interest. Maybe it was how powerful and yet how compliant to follow his orders You were, or just the plain fact that, You were just so good looking. He was a simple man, just that alone was an excuse he could use for himself as to why he had become so sickly obsessed with everything You did.
He was just so obsessed... Even when You weren't working at the League, he couldn't stop himself from following You without your knowledge. Was that creepy? Yes, absolutely, and he knew that, but he just needed to keep You on sight at all times, either by making sure to assign You in specific to go on missions with him or by down right stalking You.
He was such a sick bastard for it and he knew it but god. He just couldn't help it. He loved the thrill of the mere thought of You founding out all the sick things he imagined when he saw You, it was almost like playing a game of cat and mouse for him. Oh how much he wanted to just be around You all the time, to be able to express all his repressed affection and need for being close to You, and never letting go.
He had imagined that countless of times. Making his breath shudder just by thinking of all the things he would do to You if he could. It was a bit funny to him as well, feeling this thrill every time he stole some article from You just to have it in his room all for himself.
And that exactly, was the drop that spilled the glass, when, while searching for one of your hoodies, You found it in Tomura's bed, being used as a pillowcase, along with a lot of random objects of yours around his room...