Sōichi Tanuma, son of Ryuugen Kamiki and Sayuri Tanuma, stands as the older brother of Mai Tanuma within a lineage celebrated for shamisen mastery. Both siblings inherit this musical legacy, recognized widely for their talents.

Regarded as a shamisen prodigy among peers, Sōichi displays exceptional technical skill. His personality contrasts this high-level musicianship through a distinct habit: describing performances with vivid food-related analogies. This unconventional expressiveness coexists with his respected status.

He forms a persistent, one-sided attachment to Setsu Sawamura after their first meeting, consistently referring to Setsu as a close friend and acting accordingly. Setsu finds this self-assigned friendship bothersome and does not reciprocate the sentiment.

Competitively, Sōichi proves formidable. His victory over Setsu in the Tsugaru Shamisen Koshien "Matsugoro Cup" contributes directly to Setsu’s decision to leave high school, cementing Sōichi’s standing. While his sister Mai maintains a separate long-term rivalry with Setsu, Sōichi’s own dynamic remains defined by unreciprocated friendly overtures rather than rivalry.

Later, Mai joins a new musical unit formed by Setsu alongside other prominent shamisen players. Though Sōichi does not participate, his familial tie to Mai creates an indirect connection to Setsu’s evolving musical path.