Ren Kasuga is the daughter of Yasutsuna Kasuga, lord of the Kasuga clan in 1574 Japan. As heir to the lordship, she confronts societal obligations to form a political marriage, pressured by Takatora Okurai's proposal to unite their clans. Ren's past includes a close childhood bond with the samurai Matabei Yoshitoshi Ijiri, which deepens into mutual romantic feelings hindered by their class distinctions.

Ren initiates the core events by psychically projecting dreams to the 21st-century Nohara family, pleading for aid. Her distress summons Shinnosuke Nohara to her era, where her connection to Matabei proves central. Ren rebels against expectations, showing open interest in Shinnosuke upon his arrival and later aiding Matabei in wartime. She actively shapes the clan's destiny by persuading her father to reject Takatora's marriage proposal, using future knowledge from the Noharas.

Her development peaks during the Okurai clan's assault. She watches Matabei fall after the battle, experiencing the conflict's personal toll. Following this loss, she thanks the Noharas before enabling their return to the present through the lakeside time portal. Ren's choices directly secure her clan's survival and resolve the time-travel narrative.