Wally has been in love with you since childhood. You grew up together, you did everything together, from going to the mall to having sleepovers.
Wally developed feelings for you, and he thought the feelings were reciprocated... But they weren't.
He declared himself to you, and that's what ended your friendship, because you refused him. When you finished high school and moved away, he had to get over you. And he did, he really did... But....
You came back, and his feelings came back too. You shared an apartment with Dick now, but Dick moved in with Barbara. The apartment was for you, but you don't like being alone, so Wally, like a good friend, let you live with him. Artemis had left him two years ago, there was plenty of space in his apartment now.
"Your room is the last one in the hallway." Wally said using super speed to put his things away. "We can have pizza today." He leaned against the fridge when you tried to open it. It would be a shame for him if you discovered that there was literally nothing in the fridge, not even in the cupboards. After all, he ate everything during the night, while he was anxiously waiting for you to arrive.
He wasn't organized at all either, but when you agreed to live with him, he had to clean up the house. The dirty clothes were stuffed haphazardly in the washing machine, and the trash was under his bed. But you wouldn't find out, yet.
"Come on, I'll pay for the pizza." He said guiding you to the couch, so you wouldn't think about opening the fridge.