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
To be confirmed
Gallery
Battlecorps / バトルコープス (Mega-CD - 1994)
Cover art for the Japanese version (Source: signature found on the artwork)
Gōketsuji Ichizoku / 豪血寺一族, AKA Power Instinct (Arcade - 1993, SFC, Mega Drive - 1994)
Flyer illustration / Cover art for the Japanese version (Source: signature found on the artwork)
Gōketsuji Gaiden / 豪血寺外伝, AKA Gogetsuji Legends (Arcade - 1995)
Flyer illustration (Source: signature found on the artwork)
Jikuu Tantei DD: Maboroshi no Lorelei / 時空探偵ＤＤ ～幻のローレライ～ (Saturn - 1996)
Promotional illustration (Source: signature found on the artwork)
Ninja Master's / ニンジャマスターズ (Neo-Geo - 1996)
Main visual (Source: signature found on the artwork)
The Knights 2: Little Light From Nowhere / ナイツ2 何処からか来る小さな光 (PC-98 - 1994)
Cover art (Source: signature found on the artwork)
Shin Megami Tensei Last Bible Special (Game Gear)
Cover art (Source: )
Shin Megami Tensei (Mega-CD
Cover art (Source: )
Space Griffon VF-9 / スペースグリフォン VF-9 (PS1 - 1995)
Cover art for the Japanese version (Source: )
Battlecorps
Gōketsuji Ichizoku
Gōketsuji Gaiden
Jikuu Tantei DD: Maboroshi no Lorelei
Ninja Master's
The Knights 2: Little Light From Nowhere