As the History progresses, Rei tries to make his little sister happy, as she decided what the family was going to eat on his 9th birthday. However, he doesn't let her do whatever she wants (he made her stop running to buy the Sushi in Chapter 6). When Rei saw an Adept for the first time, he understood why his parents had told him to be wary of them.
Rei is a calm and somewhat passive seventeen year old teenager. He always tries to protect and help his sister Miu, but is sometimes overbearing to her. He is selfless and cares about other people more than himself (as shown in Chapter 15+ of Fleeting Memories).