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Kenichi's treacherous, deceitful, opportunistic amoral and generally quite sleazy friend.
Not only is Nijima a shameless opportunist with an incredibly dubious moral compass but he also absolutely revels in it, as all these traits give him a great sense of superiority.
Nijima is responsible for driving much of the series plot, especially during the Ragnarok arc, and for causing much trouble for the series' protagonist. When Nijima first appears he explains that he has a rating system for every attendee of their school, and that Kenichi, being bereft of any real positive attributes, rated rather lowly.
However, as Kenichi's proficiency in martial arts advanced and he defeated several local bullies, Nijima saw an opportunity. After spreading rumours that built Kenichi up as a brutally violent warrior who crushed anyone who got in his way, Nijima formed his own gang/organisation, the Shinpaku alliance which he alleged Kenichi was a part of so that he could use the inflated image he'd created to bolster the group's stature. He continued to manipulate and guide Kenichi into battles strategically chosen to best benefit the organisation, and moulded him into a public hero, while taking on the gang Ragnarok. Nijima's scheming took quite a bit out of the rival gang, forcing many rival members to quit, while luring several of its champions to his side.
While Nijima has no ability as a fighter, he is otherwise rather talented. He is cunning, charismatic, a great strategist, and skilled at manipulation and propaganda. He also possesses a rather bizarre ability to gauge the battle power of others by looking at them in a strange way.
Though he seems to lack any traditionally virtuous traits, Nijima displayed genuine concern and anger at the hospitalisation of many of his subordinates by Ragnarok after the rival gang grew agitated by his group's growth and actions against them. He also reveals that he's been inspired by Kenichi's beliefs and growth as a person, and hopes to use the organisation that he's formed to promote those ideals.
Nijima has a rather freakish appearance, including a forked tongue and what looks like an antenna rising from his hair, which cause people to refer to him as an alien.