He is the third male protagonist of the Science Side in the Toaru Majutsu no Index series.

    Shiage is a level 0 and former Skill-Out (with his friends Komaba Ritoku and Hattori Hanzou) after his defeat at the hands of Kamijou Touma. As former lacky, grunt, chaffeur, and informant of ITEM, he delved into Academy City's darkness and turned into an anomaly that disrupts Aleister Crowley's plans and calculations time and time again.

    Shiage can be considered a balance between Touma and Accelerator. Like most members of his Skill-Out gang, he is lawful; however, while he and Touma share some of the same traits, such as their reputations as delinquents, their victories over higher-ranked espers through tactics, and their interference with Aleister Crowley's plans, he isn't as idealistic or as nice as Touma, at least not at first. In battle, he prefers to avoid fighting if possible or to finish the fight as quickly as possible, which is somewhat like Accelerator's personality.
    Also he shows an inferiority complex that makes him think he can't do anything right or complete. The main reason behind this is that he had been always branded as a powerless person (level 0) and unlike Touma, he has yet to display any manifestation of an esper ability. Therefore, due to his lack of power and mockery by other higher level espers, he joined a Skill-Out gang. This complex can be seen in his usual internal monologue, as he always complains about the situation but he never says it out loud.

    He falls in love with Takitsubo Rikou, and because of this, he decides to fight the 4th-ranked Level 5 esper, Mugino Shizuri, also known as Meltdowner, in order to protect Rikou.

    As far the plots advances, he began to show a nicer side similar to Touma, as seen when he aids Russian villagers during pirate attacks, convinces Acqua to not give up on life, and how his reclutancy to fight Mugino leads to him forgive her and express his desire to get back everyone together as ITEM again.

    Shiage is suggestive to anything sex-related, as shown when he got a nosebleed simply by seeing Kinuhata Saiai putting a stuffed bunny doll behind Rikou to give her a bunny girl look (even though his perversion is usually used as comic relief).

o- From Wikia