TS Tensei shite Masaka no Sub Heroine ni.

A hikikomori boy who died while dreaming of a harem was reborn as one of the heroes who will save the world. The harem he aimed for would surely be there! Next to the hikikomori was someone reincarnated as the female white mage Fio, centering on the hero Alto. Fio is still crazy about girls even after her reincarnation, but the turning point comes when the party is split up by a surprise attack by the demon king's army and she and Alto take refuge in an abandoned hut.

In order to save Fio, Alto drinks a powerful potion that gives him temporary power but causes him to go into heat as a side effect... Their relationship takes an unexpected turn!