TS Tensei shite Masaka no Sub Heroine ni.

A hikikomori boy whose final thoughts were consumed by fantasies of a harem dies and awakens in a new world, reincarnated as one of the chosen heroes destined to save it. Thrilled by this second chance at life, he fully expects his dream romance to finally materialize alongside the legendary hero Alto. However, fate has a cruel sense of humor: instead of becoming the protagonist, he is reborn as Fio, a female white mage serving as a supporting member of Alto’s party. Despite his sudden transformation into a young woman, Fio’s mind remains entirely unchanged, and she continues to harbor an intense infatuation with girls. Everything shifts during a devastating surprise assault by the Demon King’s army that scatters the adventuring party. Separated from their companions, Fio and Alto are forced to seek shelter together inside a remote, abandoned hut. When Fio’s life hangs in the balance, Alto makes a desperate choice to protect her, consuming a potent potion that grants him immense temporary strength but triggers a severe side effect—sending his body into an uncontrollable state of heat. Trapped together in isolation, the dynamic between the hero and the sub-heroine takes a sudden, unexpected turn.