He is the professor who gives the lecture about time travels in Episode 1.
It is revealed in episode 23, that he is Shouichi Makise, Kurisu’s father and also the one who stabbed her in the first episode. Due to his deep-seated jealousy with his daughter’s talents, which has strained their relationship for years, Nakabachi stole her papers on the creation of the time machine and took it as his own. He later defected to Russia, and with the papers put to practice, in the future the time machine race will break out, and World War III will doom the Beta worldline.
In the Drama CD, it’s revealed that he and Akiha Yukitaka (Feiris’s dad) were students of Hashida Suzu, aka Amane Suzuha, and the three of them were devoted to the research on time machine. Shouichi changed his surname to Nakabachi, a mentor of his, in order to continue the research without being tracked down by organizations with the same interest.
Also in this Drama CD, it’s shown that Nakabachi used to love Kurisu, and was very proud of her prodigious intellect, displayed as early as two. However, as Kurisu became more intrigued by his works, she started to learn by herself and soon surpassed him. When she was eleven, she was able to disprove his time machine theory, and the shame drove Nakabachi to cut off his relationship with her, and never gave her the 11th birthday present he bought, an incident he later confessed to Yukitaka that he deeply regretted. He became more resolved to build a time machine “that could not be denied by [his] daughter”.