Yasushi Yamaguchi (also known as Judy Totoya, the nickname he was known by during the early days at Sega when their developer's identities were kept secret) is probably best known in Sonic circles for having been the character artist for Sonic 2 and the creator of Miles "Tails" Prower. But that's just the tip of the iceberg.
Not only did he create and design Tails, he re-designed Sonic for Sonic 2. Sonic's a lot more hip and cool looking in the Sonic 2 character art than in Sonic 1, where he's a lot more like a character from the classic animation era.
He also was responsible for most of the mechanical enemy designs, and Robotnik's new improved Egg Mobile.
And the Tornado, Sonic and Tails' indispensible bi-plane, which has practically become a character in the Sonic series in its own right.
Of course the Tornado is cool - Yamaguchi has always had a strength for mechanical design, as evidenced by the awesome robotic monsters he designed for Phantasy Star II.
His designs are also showcased in the geek-cool Master System game, Cyborg Hunter (Choonsenshi Borgman in Japan).
As well as working on Sonic 2, Yamaguchi was involved to a lesser extent in the creation of Sonic CD. He was a Special Stage designer, and what Special Stages they are!
The expanded data storage capacities of the CD-ROM format allowed Sega to include bonus art in Sonic CD by the designers. Yamaguchi took the opportunity to advertise his new character Tails.
(I love the fact that the licence plate says "Miles"!)
The "See You Next Game!" line probably refers to Sonic 2 - Yamaguchi most likely made the picture before leaving the Sonic CD team to begin work on Sonic 2 in the USA. Sonic CD, however, ended up being released later than Sonic 2, so it seems a little confusing - leading some to speculate that it really refers to Sonic Drift. The car beside Tails helps lend this a little credence, but I don't think there's any proof. It's not the same car design from Drift. Why the car in the first place then? Yamaguchi might have just been expressing his penchant for vehicle design.
If it does in fact refer to Sonic Drift, that might mean Yamaguchi was responsible for the vehicle designs from that game, which would make sense, judging from the style.
Back in the really early days of the Genesis, Sega had a newsletter called SPEC (Sega Players Enjoy Club), drawn by the actual game designers, such as Naoto Ohshima, Rieko Kodama, Tohru Yoshida, and of course, Yamaguchi. They all went under nicknames when drawing the issues, and Yamaguchi went by Judy Totoya (for reasons opaque to me).
Yamaguchi sometimes got cover duty, and he really did a good job, as with this Shinobi cover.
Why wasn't Sega Visions this awesome?
But Yamaguchi's coolest contributions to the SPEC magazine were his Phantasy Star manga. They were serialised, and sadly didn't get to conclude, because SPEC was discontinued. The "Basic Saga" was a humourous retelling of the story of the Master System Phantasy Star. The characters are drawn in a hilarious chibi style, and Myau looks a little like Tails...
The "Outside Saga" told of further adventures of Alis and Lutz outside the Algol system. It was drawn with a more serious tone. Seeing it makes me ache for a full-blown Phantasy Star anime series from the early 90's, but no such thing exists. Now I'm really despressed.
Yamaguchi was reunited with Rieko Kodama on Magic Knight Rayearth, but hasn't been credited since the Sega Saturn era. Where has he been? Today's games sorely miss the coolness that his personality and pen could offer, Sonic games in particular.