関智一

Seki landed his first role in the 1993 anime Mobile Suit Victory Gundam as Tomache Massarik. Some of his most notable roles include Shinichi Chiaki from Nodame Cantabile, Gilgamesh from the Fate franchise, Rob Lucci from One Piece, Gundam characters Domon Kasshu and Yzak Joule (from Mobile Fighter G Gundam, Mobile Suit Gundam SEED, and Mobile Suit Gundam SEED Destiny, respectively), Kyo Sohma from Fruits Basket, Whisper from Yo-kai Watch, Kanji Tatsumi from Persona 4, Shinya Kogami from Psycho-Pass, Ichiro Miyata from Hajime no Ippo, Haru Glory from Rave Master, Toya Kinomoto (Sakura Kinomoto's older brother) from Cardcaptor Sakura, Van Fanel from The Vision of Escaflowne, Suneo Honekawa in the Doraemon franchise, Enrico Pucci from JoJo's Bizarre Adventure, Chichiri from Fushigi Yuugi, and Ken Hidaka from Weiss Kreuz, alongside Hiro Yūki, Takehito Koyasu, and Shin-ichiro Miki.

Koyasu and Miki, along with Seki, also had voice roles in Initial D as Keisuke Takahashi, Gravitation, and Nobuo Terashima in Nana. Seki has also voiced Ryūki Shi in Saiunkoku Monogatari, recurring guest character Meito Anizawa in Lucky Star, and Shōji Tōkairin in You're Under Arrest and related media, along with Sakiko Tamagawa, Akiko Hiramatsu, and Yoshiko Sakakibara.

On November 20, 2017, Seki apologized on the online radio program Unizon for an incident which had taken place a day earlier, during an event for the online radio program A&G All Star 2017, where he had instructed Yusuke Tonozaki, a newbie voice actor from his agency, to ask voice actress Rina Hidaka to have sex with him (one of them both) as a joke.