Yuichi TAKAGI / 高木祐一, AKA You. / Back Man

Credited under his full name at the bottom of a page featuring a Last Armageddon artwork signed ‘You.’ in the July 1990 issue of PC Engine Fan.
A former colleague at Data East mentioned that he was a freelance illustrator who came to Data East with his portfolio and was hired by the company to work as both a graphic designer and illustrator. They also confirmed that he was credited as Back Man. (Sources: 1, 2)

Games
Other video game related works
Gallery

Website

Related companies:
Data East

Games (in alphabetical order)

Benkei Gaiden / 弁慶外伝 (PC Engine – 1989)
Cover art (Source: credited on the cover)

Captain Silver / キャプテンシルバー (Arcade – 1987)
Flyer illustration (Source: signature found on the artwork)

Heavy Barrel / ヘビーバレル (Arcade – 1987)
Graphic design (Source: in-game credits, as Back Man)

Makyō Senshi / 魔境戦士, AKA Gondomania (Arcade – 1987)
Flyer illustration (Source: signature found on the artwork)
Graphic design (Source: in-game credits, as Back Man)

Psycho-Nics Oscar / サイコニクス・オスカー (Arcade – 1987)
Flyer illustration (Source: signature found on the artwork)
Graphic design: ending (Source: in-game credits, as Back Man + mentioned by Akira Sakuma on Twitter)

Other video game related works

Famimaga, February 1989 issue (magazine)
Illustration: Hydlide 3 (Source: signature found on the artwork)

PC Engine Fan (magazine – 1989-90)
Illustration for several issues (Source: credited in the magazine)

Gallery