Ojousama no Geboku

After losing big at gambling, Kinoshita Hokusai finds himself buried under 1 billion yen in debt. In the midst of this, he unintentionally saves the life of Amamiya Rui, a young lady of a wealthy conglomerate, and she pays off his debt in his place. However, in exchange, he ends up serving as the sidekick for Amamiya’s personal hobby: hero activities—!?

Source: Jump+, translated