Hasegawa is the father of Konoko (Mai). Very little is explained in-game about Hasegawa as a character, though it is his actions that dictate what happened in Oni.
He was a teacher at a university (perhaps teaching biology) and fell in love with one of his students, Jamie Kerr ("Jamie Hasegawa, maiden name Jamie Kerr"). Jamie is Dr. Kerr's sister. Hasegawa and Jamie had two children, Mai and Muro.
After an unknown period of time Jamie "converted" Hasegawa to her political beliefs, convincing him that the WCG were hiding something. Images from the first cutscene of CHAPTER 11 . DREAM DIVER show several protests and Jamie participating in them. As an act of defiance against the WCG, and a quest for scientific knowledge, Jamie and Hasegawa entered the so-called "wilderness preserve", where Jamie received a scratch that turned infectious, causing great pain. The only thing Hasegawa could do was "ease her pain"; a newspaper article that has the headline "Grad Student Dies: Murder or Mercy?" is shown, telling us that Hasegawa killed Jamie.
In CHAPTER 12 . SINS OF THE FATHER, Kerr reveals that he and Hasegawa worked on the Daodan project together, but were separated during a Syndicate raid on their lab. Hasegawa's final fate remains unclear.
Hasegawa's first name is not given in any final game material. A bio created for an earlier version of the story gives his name as Kenichi, however in that story the mother's name is Barbara, not Jamie, and Konoko enters the police force at 18, so the first name is far from a canonical piece of information.