Evolt

Blood Stalk is the second-in-command of the organization Faust and one of the main antagonists of Kamen Rider Build.

His true identity is Soichi Isurugi, owner of the cafe nascita and secretly the astronaut responsible for the Skywall Disaster that separated Japan.

Past
Ten years prior to the events of Build, Isurugi was an astronaut working with the Kiwami Project. While on Mars, Isurugi discovered the Pandora Box. At the ceremony back on Earth, Isurugi applied his hand to the Pandora Box, causing the Sky Wall Disaster.

Soichi was later arrested and placed in a mental hospital, along with his daughter, Misora Isurugi who was also exposed to the box and gained the ability to purify Fullbottles.

However, Soichi escaped about a month later. Seven years later, he was appointed as a special consultant for the Pandora Box. He also co-founded Faust with Gentoku Himuro.

Under Faust
Misora later joined Faust to purify the Fullbottles for them, though she was unware at first of their true purposes. When she eventually found out about Faust's intentions, she lost her will to continue working for them. Soichi then "saved" her and took her back to the cafe, where he convinced her to continue purifying the bottles for good.

Soichi later stole the Pandora Box panels from the Touto Institute of Advanced Matter Physics to ensure their confidence and plastered into one of nascita's basement walls. He also took along a Build Driver and some Fullbottles from faustl.

Dealing with Katsuragi
When Takumi Katsuragi attempted to quit Faust, Blood Stalk knocked him out and switched his clothes and face with Taro Satou, whom Stalk had previously killed by Stalk. Stalk then left Taro Satou in the alleyway and erased Takumi's memories before talking him back to Faust to experiment on him. On September 5th, Isurugi brought the amnesiac Katsuragi to a small alleywayi After Takumi woke up, Soichi renamed him Sento Kiryu and nurtured him into Kamen Rider Build so that, with Build fighting more monsters, Soichi would have reasons to have Misora purify more bottles.