Tsukumo has an unkempt appearance, with long hair tied into a ponytail and a noticeable amount of facial hair.
Tsukumo wears a buttoned monochrome plaid shirt with a black blazer on top, complemented by grey pants with a matching silver-buckle black leather belt and black socks. He also wears high-tech spectacles and a black headset which is hung around his neck when not in use. After he moves to Yokohama, he starts wearing a pair of black shoes.
Tsukumo is quite nice and playful, sometimes demonstrating stereotypical "otaku" characteristics such as a love for rare and limited edition anime figures. He is very respectful towards those he admires, such as Yagami. He is not antisocial, but is rather reclusive, spending most of his time at his house, inside Yokohama 99, or in internet cafes. He has a lighthearted personality and is fond of playing around.
He has a seething disdain of scalpers and resellers.
Tsukumo very rarely left his house, but Takayuki Yagami convinced him to go out more, even if only slightly. Tsukumo moved into the internet café Mantai as the rent was cheap and the internet connection was strong, and made it his base of operations.
Tsukumo helps Yagami track down Aragaki using the Chatter app, and asks him to buy a rare figurine from Ebisu Pawn in return. Yagami contacts Tsukumo whenever he needs to narrow down a location for someone he needs to find, and when he needs to find special sweets for Saori. He also builds wiretaps that Yagami uses to bug the Matsugane Family office.
Tsukumo detects an odd frequency which helps Yagami track down a serial bomber. He uses his tech skills to defuse a suicide bomb the bomber was carrying as a last resort, allowing him to be arrested.
Lost Judgment: 2019-2022
Yokohama 99 receives various requests related to middle school and high school bullying. To address this, Tsukumo does comprehensive research on bullying, and calls Yagami and Kaito to Isezaki Ijincho for assistance. He gives the team hidden cameras for surveillance and hidden speakers for Yagami after bullying is discovered. When Mami Koda is bullied, three project their voices through the speakers to give the appearance that the other students are speaking, causing the students to properly defend Koda. He later provides Yagami with his gadgets.
- Tsukumo refers to certain characters (notably Yagami) with the very formal 氏 (-shi), an honorific normally used in contexts where the speaker knows of - but is not personally acquainted with - the person in question, such as newsletters and legal documents.