Niijima has a rather “alienated” appearance. Slightly tall for his age, he has a slim yet solid physique, sharp fingernails (though they seem to be deleted at some scenes), and long gangly limbs. His facial features are somewhat demonic: a long face with sharp nose, long pointed ears (a fact pointed out by many characters), a long and forked tongue, and glaring eyes. Depending on his emotional state, his teeth are either depicted as jagged, fanged, or even just normal. Not surprisingly, Kenichi and many other characters refers to him as a "demon-faced space alien" or something similar. He originally sported puddle-bowl hair, but later allowed the back to grow sideways.
At times, when he is in a state of fierce concentration, two pointed antenna will pop out from the top of his head.
Almost every time, he wear his school suit as his "battle" (or escape) outfitt. He usually wear a cloak in base of Shinpaku alliance to claim his leadership and his "Demon King" image, during D of D Tournament, the cloak he wear is a new flag which painted label and kanji of shinpaku alliance. When it come to some ceremony event such as party held by himself and captains of shinpaku , which come with the assault of berserker.
Though Niijima was initially seen as both a comic relief character and an ambitious power-grabber, as the series progressed, it becomes apparent that he is actually more perceptive and empathic than he lets on and is an altruist at heart, with an almost paternal air of leadership towards his subordinates. His proclaimed dream of one day ruling the world remains, but it is very clear that he prioritizes the well-being of the organization that he set up and everyone who is a part of it, even if it meant withholding from them crucial information and risking (and often resigning himself to peril for their sake). He is very adept at running away from dangerous situations, always leaving the fighting to the others, a trait which Kenichi adopted to "face" women in battle. Despite his often cowardly tactics, he shows himself to be a very cunning individual, thinking many steps ahead and planning for contingencies to the point that both the enemies and allies have complimented on his leadership skills.
He is a very sly person who watches Kenichi's progress with much interest (and keeps his information up to date by bribing Miu with anmitsu, her favourite food) and treats Kenichi as his subordinate. He is very adept at running away from dangerous situations and can even outpace the likes of Kenichi, who can move considerably fast thanks to Ryozanpaku's training, though it only works while he is running away from somebody or something.
Throughout the series, Niijima has proven to be a very skilled escape artist, having shown to be a capable swimmer (he used this method to survive a direct confrontation from Hermit), and a talented dodger (he frequently evades fighters whose fighting skills rank fairly high, and his greatest achievement yet in this regard was seen when he dodged, along with Kenichi, a direct sword strike from a master-class member of Yami's armed division while on top of a speeding truck that cleaved the rear of the vehicle in half). A fact made apparent where he repeatedly thwarted Berserker's (one of Ragnarok's top three) attempts to ensnare him by deliberately stripping his clothes piece-by-piece every time Berserker thought he caught him, and during the DoD Tournament where, after volunteering to be part of the contending players representing the Alliance (since Freya, Thor and Takeda were crippled by Shou, leaving them only with Kisara and Ukita), Niijima more than played his part where he held the captain of the opposing team (long enough for Kisara to beat the remaining opposing fighters) at bay by keeping the distance between them through the use of continuous dodging and running, along with the clever use of minor ambushes. It is also worth noting that after the fight, Niijima alone was the only person from the Shinpaku members who took part in the ordeal and got off practically unharmed.
Despite his often cowardly tactics, he shows himself to be a very cunning individual (having once saved the Ryozanpaku dojo from certain bankruptcy after devising a legitimate get-rich-quick scheme, which a desperate Miu adopted at once), thinking many steps ahead and planning for contingencies to the point that both the enemies and allies have complimented on his leadership skills. He is also a skilled tactician, capable of coordinating the combat formations and ploys that the Alliance use at both in and out battle (such as postponing the battle between Kenichi and Chikage by convincing a classmate to invite the latter into a birthday party).
Despite the fact that he, as well as most characters, calls himself evil, he usually finds ways of helping his friends when needed, though he does so in his own odd way. As the series progressed, his character is seen developing from an ambitious and conniving opportunist to an altruistic, understanding and almost paternal leader who, while seemingly will stop at nothing to achieve his goals (and often subsequently getting a comical beating from those who simply find him and his ambitions to be annoying), is actually empathic towards those under his control, and considers their well-being above all else, even if it meant withholding from them crucial information (like not telling Kenichi that another Yomi was in the same school in the form of Chikage to prevent the two from seeking a bout with each other) and risking danger himself for their sake (as seen when he repeatedly provoked Berserker to lure him away from Ukita and Takeda) and even forfetted his team from the DofD Tournament to protect them from getting killed by Sho Kano, something Shigure said is the true capabilities of a leader. For the members of the Alliance itself, they appear to have developed genuine respect towards Niijima overtime as their unquestioned leader, even Kenichi as well as the captains, since their repetitive successes (be it victory or survival-wise) in battle have mostly been influenced by Niijima in one way or another.
While generally a hands-off non-combatant who prefers to keep his distance and coordinate attacks, if forced to exert himself in battle, Niijima usually relies on hit-and-run (mostly run) and psychological tactics to unnerve his opponents and render them more vulnerable, and lets the more powerful fighters of Shinpaku to deal the finishing blow. He has a (more or less) finishing move called "Shadowless leap from the charge", which he uses to escape by switching positions with a nearby enemy. He has amazing information gathering and analytical skills (to the point that he can quickly identify a master-level fighter in a very short period of time), which he uses to prepare Kenichi for the many battles that he ends getting tangled into with his opponents' profiles and skills. He has trained the non-combat members of his "alliance" fiercely in the art of gathering information.
Niijima's ability to collect data is chillingly effective, and in some cases, supernatural, such as the bizarre "Niijima Scan" technique that enables him to absorb ridiculous amounts of unknowable things about a person (Physical, Personal, Mental, Emotional, Skill Level, all of it...) simply by observing them for a few seconds (He was able to tell that a member of an opposing group had a crush on his ally through eye contact alone). What's more, he can ring his thumb and index fingers around his eyes to literally create a digital HUD of his target's power level and killing intent, and even has a zooming function, allowing him to view from afar as though he's looking through a pair of binoculars. Apparently, his senses are inexplicably unnatural, having once detected incoming missiles locked on to their location, and even discerned the presence of soldiers hiding in the woods to the point where even a Yomi member, Boris Ivanov, commented on Niijima's sensory skills that he immediately ordered his subordinates to distance themselves away from the Shinpaku leader. During the Eternal Sunset Arc, after giving an analysis on Kajima Satomi, the Yomi leader found his evaluation so accurete that he deemed Nijima a very dangerous individual, leading to him attempting to take Nijima's life then and there.
Niijima also has a talent for both tinkering and programming, having once constructed a working desktop computer from spare parts and even created the official web page for the Shinpaku Alliance with considerable security (as Loki found out when he tried to hack the page). He is also skilled engineer and traps master, having constructed rudimentary traps to ambush the Capoeira team captain, while at the same time running around the arena (at high speed), and even took part at supervising the renovation of the Shinpaku Alliance hideout, not to mention the traps that were stationed at Kisara's old hideout (now the Shinpaku's official base) to safeguard the place from infiltration. He is also very adept at espionage, and can occasionally be found casing the scene (with Matsui as his second aid, of course) already before the waited event takes place; he even once stole the keys to Tanimoto's house without him even realizing it (and has apparently made duplicates, granting limitless access to Tanimoto's property indefinitely, much to the latter's chagrin) and, while in a drunken state, once single-handedly hijacked a cargo ship all on his own.
Perhaps his greatest accomplishment in this regard yet was the fact that he once spied on the masters of Ryozanpaku and actually remained undetected for a time, and by the time Shigure sensed his presence, Niijima swiftly switched places with one of Akisame's training dummies, allowing a clean escape. He can even hide his presence from people by pressing his body against a shadowy area, using his bizarre alien abilities to render himself undetectable (though exactly how this works can be anyone's guess). He seems to have access to hammer-space, since he has been known to pull out convenient items (i.e. pepper sprays, striking batons, and of course, his prized PDA) from under his clothes. Niijima is also very perceptive, and has, on several occasions, perceived minor details that Kenichi himself overlooked. His potential tempted Fortuna enough to try to convert him into one of his children even though he only takes kids with the potential to reach master class. And as stated by Kenichi himself, Niijima has a natural talent at manipulating people (either through persuasive language, or even blackmail, etc), which would explain how he was able to form a prominent group of diverse martial artists like the Shinpaku Alliance.
He also has a problem entering churches and other sacred places, as seen when he tries to enter the church where Loki had Kenichi's sister captive and becomes weakened to the point where he must be carried. As he was being, carried his shadow could be seen struggling and screaming, and his body smoking. He's also somewhat cowardly in situations where he has to defend for himself, as he tipically has to rely on other Alliance members to protect them.
Niijima seem to feel kinship with those who can read/predict other people thoughts as he called Tirawit Kōkin a fellow mind reader.