Square Smash

The Square Smash appeared in 2017 TV series called Kamen Rider Build. He's actually Masahiro Nabeshima (鍋島 正弘 Nabeshima Masahiro) who is a prison guard working for Faust. Later he became one of the victims of Faust's experiments who got turned into the Mirage Smash (ミラージスマッシュ Mirāji Sumasshu) and later the second  Square Smash  (スクエアスマッシュ Sukuea Sumasshu). His Smash essences were used as the basis for the Ninjya (Mirage Smash) and Comic (Square Smash) Fullbottles.

Little is known about Nabeshima aside from his role in Ryuga Banjo's life. At an unknown point in time, Faust forced Nabeshima into joining them, threatening to kill his wife and daughter if he refused.

On the orders of Blood Stalk, he approached Kasumi Ogura, Ryuga's fiance, and had her direct Ryuga to Takumi Katsuragi in exchange for helping Ryuga make a comeback as a fighter. When Ryuga arrived to meet Katsuragi, he was framed for the latter's murder and imprisoned, where Nabeshima sedated him and brought him to be experimented on by Faust.

Following his escape, Ryuga managed to contact Nabeshima, calling him to demand a confession. Nabeshima revealed to Ryuga that the organization that experimented on him, Sento, and the Smash was called Faust, and that they were holding his family in Seito, saying he would testify against Faust if Ryuga rescued them. However, this information was delivered on Night Rogue's order to lure Ryuga to Seito. Nabeshima demanded his family be freed in exchange for playing along; instead, his reward for his service was being turned into the Mirage Smash. He attacked a group of Touto Guardians and Sento, fighting Build RabbitTank by duplicating himself. After Build switched to HawkGatling and defeated him and all his duplicates, his Smash essence was removed into a blank Full Bottle. However, he was taken away by Blood Stalk after being recognized by Sento.

Nabeshima was injected with Nebula Gas again, transforming into the Square Smash. He was set upon a park, coming to blows with Build once again. After fighting for some time, Blood Stalk fires upon him with his Steam Rifle, causing him to grow in size. Build proceeds to defeat him with the Full Bullet finisher, and upon having his Smash essence extracted returns to normal once again.

Some time later he wakes up in Nascita's basement lab. However, the second Smash transformation has robbed him of all his memories, unable to even recognize his wife and daughter. Shortly after the reunion he goes over to the panel with the Fullbottles plugged in, suddenly remembering that Faust had similar bottles as well.

Sometime after Faust's supposed dismantling, Nabeshima managed to recover some of his memories, calling Sento to tell him who told him to pin Takumi Katsuragi's murder on Ryuga: Blood Stalk, also known as Soichi Isurugi.

Sometime before the proxy fight between Touto and Seito to determine the outcome of the war over Pandora's Box, Nabeshima's family was kidnapped by Namba Heavy Industries, and were used by Juzaburo Namba to pressure Sawa Takigawa into stealing data on Build's new power up item. However, their plan would end in failure as Sento gave Sawa half the data in order to buy time for Kazumi Sawatari to find and rescue Nabeshima's family.

Complete

 * Block Creation: The Square Smash can cut blocks from any material with its knife right arms. It can throw these blocks to attack opponents or raise columns of them for defense.
 * Enhanced Jump: The Square Smash can jump long and high distances.

Giant

 * Enhanced Strength: After being enlarged by Blood Stalk, his strength is increased, although he loses his agility.