Tatsumi Sekiguchi is a crime fiction writer grappling with financial instability, contributing frequently to news magazines. His physical appearance features a small stature, poor posture, heavy beard growth, and a rough, ape-like facial structure contrasted by notably long eyelashes. He experiences excessive, waterfall-like sweating. Adaptations occasionally depict him wearing glasses, though this detail is absent from the original novels.

Sekiguchi studied slime molds in university before being mistakenly drafted into military service despite exemption. He served as a student officer and squad leader during wartime, where Shutaro Kiba was under his command. Their entire squad was destroyed, leaving Sekiguchi and Kiba as sole survivors. This fueled psychological trauma manifesting as depression, anxiety, and possible dissociative tendencies. Post-war, he abandoned science due to financial hardship and turned to writing. He publishes under his real name and the pseudonym "Itsumi Soki," remaining obscure in literary circles with only one recognized book.

Profoundly shy and socially inept, Sekiguchi exhibits worsening depression when entangled in disturbing events, sometimes becoming nonverbal under extreme stress. He harbors deep insecurities and inferiority yet possesses an underlying internal fierceness. Easily swayed by rumors, supernatural claims, and assertive figures like Soichiro Enokizu or Morihiko Toriguchi, he avoids religious matters due to his grandfather’s cult involvement that ruined the family financially. He lacks strong convictions in science or spirituality, struggles with communication—often resorting to overly literary phrasing—and acquiesces to others to avoid conflict, occasionally employing minor, non-malicious deceptions.

Married to the supportive Yukie Sekiguchi, he receives occasional writing assistance from Akihiko Chūzenji’s relatives. His relationships with his educator parents and younger brother are distant. He maintains complex ties with university acquaintances Chūzenji—who publicly calls him a mere acquaintance—and Enokizu.

Sekiguchi holds no investigative aptitude. His knowledge spans slime molds, mycology, neuroscience, psychiatry, and psychology from academic and personal mental health struggles. He possesses cultural and culinary knowledge despite limited finances and poor taste. Enokizu compelled him to learn bass guitar, though he lacks rhythm or musical ability. Sekiguchi often stumbles into criminal investigations, sometimes becoming a suspect due to proximity or case connections.

Additional traits include pretending to smoke without inhaling, a reputation for extreme misfortune, and residence roughly 20 minutes from Nakano Station. He is deceased by the 21st century within the story’s timeline.