Genos has had feelings for Saitama for a long time, and those feeling are what persuade him to start bringing the class-B hero along on his missions. After all, who wouldn't want to have their crush watching them fight?
Saitama is overjoyed when Genos offers to take him on missions. But more feelings arise than expected while watching the Demon Cyborg, because Genos' fighting is amazing, and frankly, quite a turn-on....
Cover art by kukumomo on Deviantart.