Professor Hasegawa has been teaching Kamen Rider history at the local high school for decades. He befriended a teenage immigrant from South Korea named Bae, who was one of his star pupils. The boy had a knack for understanding the process, and started developing working devices of his own--though none were very powerful or long-lasting. Hasegawa was excited, and suggested Bae enter the device in a scholarship competition being held by a local scientific organization. Bae did, but on the way to the actual competition he and his technology disappeared without a trace.
A year later he returned, but he was a totally different person—traumatized, paranoid and full of crazy-sounding theories. What he talked about intrigued the professor, who became his confidante until his therapists convinced him to stop talking crazy talk. They lost contact for a while.
Hasegawa first met Amaya during detention, when she got distracted from her homework by one of the blueprints he was designing. She'd noticed a mistake in his calculations and corrected it. The two became good friends, particularly after they discovered their connection.
Professor Hasegawa is excitable and rather flighty. He tends to flap his arms as he moves, and he’s very untidy—differently organized, he insists. Forgetful, skips from one thing to another, except when it comes to his passion—Kamen Riders. Then he becomes intent, passionate and even brilliant at times. Mostly he’s very amiable and socially unaware, especially when he’s being mocked.
The professor goes out of his way to make sure Amaya doesn’t do anything dangerous, not wanting a repeat of what had happened to Bae. He still blames himself for that incident, so he stifles his otherwise overpowering curiosity whenever Aaron brings up something that could put him at risk. He worries about his young student, though still forgetful and easily distracted when it comes to anything but the Kamen Rider stuff.