Yuki Hiiragi

She may hold the same last name as Youta, but Yuki actually lost her memory and thus, has been staying at Youta's house for the past six months. Not only is she bright, but she is quite straightforward; she has been a tad out-of-touch with society due to her memory loss so she can come off as clumsy at times. Yuki seems to be quite interested in historic ruins and ancient civilizations.