Seiichirou Takamine holds the position of second-in-command within the Yamainu Squad, a unit specializing in combating supernatural threats. Aged 36 and standing 183 cm tall, he wields a large cleaver-shaped sword. His personality merges a quick temper with a strong sense of duty, leading him to enforce strict discipline among subordinates, sometimes influenced by his fondness for alcohol.

Takamine shares a long professional history with Yamainu captain Abe no Sousei. His leadership involves rigorous training and punitive actions, exemplified when he disciplined squad member Takeda Rakucho for aiming a weapon at civilian Tenka Kumo. Despite this severity, he mentors Takeda, accepting sake as payment for training and showing concern for colleagues during crises.

During the Orochi conflict, Takamine participated in key operations, including the arrest and presumed execution of Tenka Kumo. He later sustained severe injuries during an Orochi attack near the Kumo residence, resulting in hospitalization. This incident triggered visible distress in Takeda, revealing squad loyalty. Following the Orochi's defeat and the Yamainu's disbandment, Takamine survived but faced an uncertain professional future alongside former comrades.

In the Gaiden storyline one year post-conflict, he remained involved with the group investigating ongoing Orochi-related experiments. His interactions with Tenka Kumo, stemming from a past acquaintance through Sousei, were defined by institutional obligations rather than personal rapport.

Takamine's character development maintains consistency in his disciplined approach and tactical reliability, with no significant alterations to his core traits or role across the series, films, or spin-off media.