Chances are Seijin is a pen name used by Haruhito TOMOBE. Seijin is presumably credited as Tomobe Seijin in Ninja Master's. Haruhito TOMOBE is credited as illustrator in Jikuu Tantei DD: Maboroshi no Lorelei, one of whose illustrations is signed "Seijin".
Games
Gallery
Hyper Pro Yakyū '92 / ハイパー・プロ野球'９２ (Game Gear - 1992)
Cover art (Source: Sega TV Game Genga Gallery)
Jikū Tantei DD: Maboroshi no Lorelei / 時空探偵ＤＤ ～幻のローレライ～ (PS1, Saturn - 1996)
Manual illustrations (prologue illustrations) (Source: in-game credits)
Mephist / メフィスト (PC-88, PC-98 - 1989)
Cover art (Source: signature found on the artwork + credited as "package designer" in the game)
Pyramid: Cleopatra Kiki Ippatsu / ピラミッド クレオパトラ危機一髪 (Famicom - 1990)
Cover art (Source: credited on the back of the box)
Hyper Pro Yakyū '92
Jikū Tantei DD: Maboroshi no Lorelei
Mephist
Pyramid: Cleopatra Kiki Ippatsu