Nariaki Utsumi

"I just wanted...to laugh at trivial nonsense...to cry over at misery...to live a simple life like that."

- Utsumi's confession about his ideals his life

Nariaki Utsumi (内海 成彰) is the assistant of Gentoku Himuro at the Touto Institute of Advanced Matter Physics. He is also an aide to the Prime Minister of Touto and a member of Faust.

He is later used as a decoy in order to hide the true identity of Night Rogue during Touto's operation against Faust, before being gunned down by Gentoku himself.

Character History
Utsumi is first seen reporting to Gentoku about Sento's application for a job at the Touto Institute of Advanced Matter Physics, noting that Sento had passed the entrance test with a perfect score.

Utsumi is Sento's direct superior at the Institute, reporting directly to the Prime Minister of Touto, Taizan Himuro, while at the same time also secretly assisting Gentoku/Night Rogue within Faust. He was one of the first present witnessing Taizan's collapse, after the Pandora Box was stolen from the Institute. The Box was later hidden within a secret basement at one of Nanba Heavy Industries' laboratories.

Shortly after the emergence of a second Kamen Rider, Utsumi was called upon by Jusaburo Nanba for a favour. Later, as he watched Sento return to work despite it being a day off for all lab staff, Utsumi decided to reveal the location of the Box's hidden location to Sento. Though, the Box ended up being reclaimed by Blood Stalk, and Utsumi was eventually forced to cooperate with Nanba in his plan: to assume the identity of Night Rogue as a decoy in a staged operation by Touto to supposedly take down Faust. Due to Build's intervention, Utsumi was seemingly exposed as Faust's creator, though Build suspected that something was amiss.

Chased by Build to a bridge above a river, Utsumi, at his wits' end, confessed his inner troubles as a scientist. Revealing that he already knew that Build was Sento for a long time already, Utsumi urged Sento to not land into the same fate as his, and warned him to 'turn back' when he still have the time before he was gunned down by Gentoku, causing Utsumi to fall into the river below to his apparent demise.

Utsumi's ultimate fate is currently unknown.

Personality
Nothing is much known about Utsumi's personality other than being a loyal assistant to Gentoku, even when his superior is revealed to be the leader of Faust and manipulating the Touto government from behind the scenes, as shown smirking whenever Gentoku, as Night Rogue fighting Blood Stalk. Though, he does seem slightly hesitant and reluctant at times as well.

It is later revealed that there is also a hidden depth to his personality, as Utsumi admits to Sento that they have a lot in common whenever they are trapped in unfitting situations they are in. He wanted to live a simple life as a scientist, though he admitted that he had lost his way as time passed, and warned Sento not to fall down the same path as him as a fellow scientist.

Forms
"Bat! Mistmatch! Bat...Ba-Bat... Fire!"

- Transformation announcement

Ability Perimeters: Like Blood Stalk, Night Rogue's form is accessed with the Transteam Gun and the Bat Fullbottle.
 * Height: 197.5 cm.
 * Weight: 103.0 kg.
 * Punching power: 17.2 t.
 * Kicking power: 20.5 t.
 * Maximum jump height: 49.0 m.
 * Maximum running speed: 100 m. per 4.1 sec.

Night Rogue seems to possess a degree of steam manipulation as he can turn himself into steam to quickly travel long distances or to summon his Steam Blade. Night Rouge is also shown to be able to fly as well as stand upside down by using his feet to somehow cling to ceilings like a real bat. He can also execute an electrified Rider Punch-esque attack where his fist glows blue with electric energy and hits the target.

Unlike Build, Night Rogue's Ability Perimeters are not spread out between his arms and legs. While Night Rogue gets power from only one Fullbottle, he's much stronger than Build's Trial Forms and his stats even surpass some of Build's Best Matches. He is also able to overpower Blood Stalk in one-on-one combat, despite having marginally lower stats.

With the Steam Blade or the Steam Rifle, he can use the following attacks:
 * Elek Steam (エレキスチーム): Night Rogue shoots electricity at the enemy.
 * Ice Steam (アイススチーム): Night Rogue performs a series of icy slashes at the enemy causing them to freeze.
 * Devil Steam (デビルスチーム): Night Rogue fires a "steam" bullet which will home in on the target and inject them with an unknown compound when it hits them. When used on a human, the person will turn into a Smash. When used on a Smash, the Smash will grow to gigantic proportions.

Night Rogue has 4 finishers:
 * Transtream Gun finishers:
 * Steam Break (スチームブレイク): Night Rouge channels the powers of a Faust Fullbottle and performs a powerful blast attack.
 * Bat:
 * Steam Attack (スチームアタック): Night Rouge channels the powers of a normal Fullbottle and performs a powerful blast attack.
 * Steam Rifle finishers:
 * Steam Shot (スチームショット): Night Rouge channels the powers of a Faust Fullbottle and performs a powerful steam blast attack.
 * Bat:
 * Steam Attack (スチームアタック): Night Rouge channels the powers of a normal Fullbottle and performs a powerful steam blast attack.

Appearances: Build Episode 12

Devices

 * Transteam Gun - Transformation device and personal weapon.
 * Fullbottles - Transformation trinkets.

Weapons

 * Steam Blade - Standard sidearm weapon. Can combine with the Transteam Gun to form the Steam Rifle.

Portrayal
Nariaki Utsumi is portrayed by Yuki Ochi (越智友己).