Sōichi Isurugi is a major character and the true identity behind the villainous Blood Stalk in the tokusatsu series Kamen Rider Build. Initially introduced as a friendly café owner and a mentor to the protagonist, he is later revealed to be a cunning and manipulative antagonist. As Blood Stalk, Sōichi orchestrates many of the series' conflicts, playing a pivotal role in the development of the plot and the transformation of other characters through his complex schemes.