Reiji has hair that seems to be uncombed or has no particular style. His eyes are dull-looking, likely since Shizuru passed away, and has visible eyebags beneath, potentially caused by him focusing too much on his work. He also possesses a growing stubble as a facial feature.
Reiji is a very focused individual, as shown during one of his lectures where he continued teaching while reprimanding both Rizu and Sawako in the middle of it. Due to his focus and love of mathematics; he is amazed at any person who displays any sort of aptitude or who he believes are worthy to study it, as shown when he praised Rizu for her work in it. It is also what led him to fall in love with Shizuru, deeply believed she is a genius.
Similar to Mafuyu, Reiji is a firm believer that people with a talent for a specific field should make great use of their skills on it, rather than on something they obviously can't. This is shown by his lack of concern for Fumino (Reiji's own daughter), who wishes to continue studying science and become an astronomer. Because of this, Reiji constantly shoots down his daughter's dream due to her poor grades, calling her efforts "childish" and that she is not worthy.
He also went as far as to tell Fumino that if she continued to persist that he will not take responsibility for her and that she should leave their house and live on her own.
However, it is later shown that Reiji, deep down, does care about his daughter as in his private conversation with Hanae; he reveals that he feels ashamed for having once slapped Fumino without thinking and had been unable to properly face her ever since. Additionally, he also greatly loved Shizuru for showing him the "emotions" behind mathematics, something which he still carries with him today.
He later reconciled with Fumino after viewing Shizuru's video, the apologies to Reiji, that she lied to Reiji that she had a journal paper for Millennium Prize Problems. He was surprised that Shizuru was poor in mathematics in high school. He also accepted Fumino's preferences after reconciliation. By the way, he concerns the relationship between Fumino and Nariyuki.
- The name Reiji means "zero" (零) (rei) and "samurai" (侍) (ji).
- Reiji's surname Furuhashi means "old" (古) (furu) and "bridge" (橋) (hashi).
- Reiji and Shizuru studied mathematics in university where they both obtained doctorate degrees.