|Super Ultra Spoiler Warning: Reading this before watching Neon Genesis Evangelion will ruin your life. Well, you'll regret it, at least. This is only intended as a review and clarification of Gendo's role in Eva for those who've already seen the show.|
Gendo Ikari is the director of Nerv -- the last, best defense against the Angels that have come to destroy humanity. He has no time for pleasantries or friends; he's intent on his work, and he's a cold, calculating SOB. Gendo Ikari is also the imposing, forbidding father figure that Shinji runs away from before the start of the series. We come to know him through Shinji's eyes: he's disapproving of Shinji; Shinji's very existence seems to be an inconvenience, a distraction from Gendo's all-important work. Only when Gendo needs a pilot for Unit 01 does he send for Shinji, and even then he will only deign to talk down to his son from a great distance in order to see if Shinji will risk his life to defend the city against an unknown monster.
Gendo also has a girl of Shinji's age working as one of his pilots, with whom he seems to share a special bond. She first showed up a year after his wife died under mysterious circumstances. Eventually the ambiguity is dispensed with and we realize that he's been cloning his wife (using somewhat unconventional "technology"). Not only that, but his whole goal in bringing about Instrumentality is to bring back his real wife; it turns out that love for Yui Ikari was his overriding motivation. We even come to see that he cared about his son all along, but was unable to express his feelings.
Eventually, Shinji has to confront his father and his father's opinion of him, in order to complete his hero's journey. He decides that he wants to please his father (and also to defend his friends), and so he does his father's bidding, and finally receives Gendo's approval by becoming a stronger person, accepting the world he lives in, and even learning to like himself. Nevertheless, for most of the story, Shinji's father is an overpowering figure that stands as a sort of psychological bogey, an obstacle to Shinji's egoistic development from a child to a young man. The elevator scene in the image on the right is a depiction of that aspect of Gendo Ikari (click the picture for a summary of the scene).
Gendo and Hasegawa
Gendo and Professor Hasegawa both represent humanity's last line of defense against a worldwide peril. They are brilliant, they know that there's a threat to Earth's future, and they know how to avoid it. Also, they both lose their wives fairly early on and are driven by that loss to work harder on their research/experiments, perhaps to the exclusion of commonly-accepted morals (a difference being that Gendo apparently alters his previous goal for Instrumentality so he can bring back Yui, whereas Hasegawa is only made aware of the threat to Earth when he loses Jamie).
These similarities could seem like a coincidence, but both Brent Pease and Hardy LeBel stated that Evangelion was one of their favorite anime. In fact, Hardy considered it to be the best anime, and explicitly stated Evangelion was an influence here. Beyond the plot similarities, we can't say how much Hasegawa is similar to Gendo because Hasegawa's character is not elaborated on in Oni. Is Hasegawa a cold man like Gendo, is he driven by grief, or is he warmer and more likeable? Oni gives us almost nothing to go on, but it's difficult not to picture him as at least somewhat similar to Gendo Ikari, considering the connection the characters have.