Underneath It All, Ren is a Woman

Overview

Company employee Tokiwa lives with his cousin Asagi. The two of them were living their normal, comfortable lives until one day, intoxicated with liquor on the night of Tokiwa's birthday, they began to reveal their deepest secrets... "The truth is, I remember my past lives." "The truth is, I'm a time traveler." A science-fantasy tale in which the fates of a reincarnation and a time traveler are intertwined! Note: Chapter count includes four extra chapters, an epilogue, and two volume bonus chapters.

Source: AniList