Katsuhiko Jinnai (陣内 克彦 Jinnai Katsuhiko), manages to win the Student Body Presidential election by promising student government funds to campus clubs and organizations that voted for him.
When Makoto catches wind of this and plans are made for him to testify against Jinnai (who is called "Jinnai" by everyone except his sister), the egocentric, power-hungry and soon-to-be-impeached Student Body President attempts to thwart the process by attacking Makoto, intending to keep him tied up until a technicality in the bylaws prevents him from being thrown out of office. Before he can execute his plan, however, he's sent to El-Hazard, and once there encounters the Bugrom, a race of human-sized insectoid beings ruled by Queen Diva.
Jinnai's power granted to him by the trip is the ability to communicate with the Bugrom, allowing him to become their military commander under their belief that he is the messenger from God sent to lead the Bugrom to victory. A skilled tactician, he is able to reorganize the Bugrom forces into an effective army and comes close to conquering El-Hazard.
Although Jinnai is a power-hungry egomaniac, he's not without some level of humanity. When Galus attempts to stop Makoto and the others from halting the Eye of God's apocalyptic malfunction, he takes the leader of the Phantom Tribe by surprise with the help of a Bugrom soldier. Although Jinnai effectively comes to Makoto and Nanami's rescue, he does so more for the fact that a "true ruler doesn't destroy the land he wishes to conquer." Jinnai has a very distinctive laugh.
Jinni suffers from an obsession with Makoto that borders of the psychotic. Over the course of both series, he consistently blames Makoto for essentially every single failure and disappointment he experiences. His sense of his own history is a montage of misfortune, the chief architect of which is Makoto. Though generally megalomaniacal, one of his primary motivations in gaining power is to subsequently gain the ability to best, or, even kill him.
Jinnai in The Wanderers
Jinnai does not change much in the transition from OVA to TV series, either, but he seems to treat Ifurita with more care. Although he's threatened with impeachment from his position as Student Body President at the beginning of the series and Makoto plays no direct role in the trial, he is paranoid that Makoto will replace him as president once he is kicked out of office.
Additionally, Jinnai reveals a softer side at the end of the series, showing a certain fatherly affection towards Ifurita after she stops the Eye of God from plowing into Roshtaria. ~From Wikipedia
