United Game Artists

Tetsuya Mizuguchi joined Sega in 1990 as a designer of arcade cabinets. Prior to entering the video game industry, Mizuguchi majored in literature at Nihon University's Faculty of Arts. Asked how he chose a career in video games, he explained, "I preferred doing something in relation to human senses or entertainment - something more in relation with human nature, a field where I could do some research.... Unlike the arts, where it is often a matter of taste whether something is good or not, creating good interactive entertainment is more easily definable. I chose Sega because it was using new technology and I was able to study things like human movements." Mizuguchi has expressed that he originally had no interest in making video games and wanted to be involved in other areas of entertainment, including creating theme parks and attractions. During his time at Sega, Mizuguchi developed an interactive 'ride' titled Megalopolis combining then-embryonic 3D polygonal graphics and CGI with the physical experience of Sega's hydraulic 'AS-1' motion simulator. He went on to produce the racing simulator Sega Rally Championship and Manx TT Super Bike.

In 1996, while working as a producer for Sega's AM3 division, Mizuguchi met with Hisashi Suzuki, the manager of the division. He and Mizuguchi agreed to create a new department separate from AM3 that would be called AM Annex. Mizuguchi selected the initial team himself, a team of six or seven people that would later grow in number. The first game AM Annex began to develop was Sega Touring Car Championship on the Model 2 arcade board, and Mizuguchi also contributed to Sega Rally 2. According to Hisao Oguchi, later head of AM3, Mizuguchi and developer Kenji Sasaki had departed AM3 with the team of Sega Rally Championship, and Mizuguchi later chose to leave Sasaki after being granted their own AM department and set up development in Shibuya. Sasaki became head of AM5, later renamed Sega Rosso.

Mizuguchi's department in Shibuya would be known as Sega Consumer Development 4 (CS4) and Sega Research and Development #9 (R&D #9), also known as AM9. During this time, the department developed Space Channel 5 for the Dreamcast. Mizuguchi had been tasked by Sega with creating a video game that would have a broad enough appeal to draw in casual female gamers. In order to achieve this, Mizuguchi conducted interviews with girls to find out what they liked. He insists that it is difficult to create a game that appeals to both genders, due to different desires in games. During development of Space Channel 5, Mizuguchi noted one stage in development where the game was "cool, but not so fun", and so he tried to introduce elements inspired by Stomp. In order to lighten the mood with his staff, Mizuguchi invited a mime artist to visit the department and help his team to loosen up.

In 2000, Sega separated its development divisions into subsidiary companies, fully owned by Sega. R&D #9 became United Game Artists (UGA), and Mizuguchi was installed as its CEO. According to Mizuguchi, the name was one he wanted to use for a video game studio for some period of time. Mizuguchi set a goal with UGA to create games for a worldwide audience, but not with any particular genre in mind. In transitioning from racing games to music-based games, Mizuguchi noted that while racing games were a good way to show off CGI graphics, the release of the Dreamcast gave opportunities to engineer better music, and he wanted to incorporate interactivity with music in games.

UGA's next title would be Rez, a musical rail shooter. Mizuguchi initially considered the concept of a video game as an art form after playing Xenon 2 Megablast. Later, while traveling Europe in 1997, Mizuguchi had been taken to the Street Parade in Zurich, during which there was a large electronic dance music concert attended by around 300,000 people. Mizuguchi was taken in by the sights and sounds around him and recognized how this experience was similar to the inspiration that Wassily Kandinsky, a Russian painter, had channeled to "[paint] a canvas of the sounds that he saw". He saw this tie to his previous ideas and envisioned a game where one would shoot down enemies in time to the beat of music that would put the player into a trance, using a theme of synesthesia, forming the basis of Rez. Mizuguchi and his team began to research how to structure this game, visiting clubs and attending Taiko drumming festivals. On pitching Rez to Sega executives, Mizuguchi has expressed that presenting the game was difficult until management was given the opportunity to try the game, due to difficulty in describing the game's concept in words. Rez was well received upon release, as were its re-releases on later consoles.

The final release for UGA would be Space Channel 5: Part 2. According to Mizuguchi, despite low sales of the original game, Part 2 was easier to pitch than Rez had been due to there being an established base of fans of the original, as well as the prominence of mobile phones and MTV. Part 2 was designed using real-time graphics instead of the pre-rendered backgrounds of the original game in order to make the game appear more like a movie. Space Channel 5: Part 2 was released in Europe and Japan, but would not make its debut in the United States until the release of the compilation Space Channel 5 Special Edition for the PlayStation 2.

In September 2003, Sega performed an internal restructuring of its staff. Among these changes was the dissolution of United Game Artists and the transfer of its members into Sonic Team. The following month, Mizuguchi left Sega. According to Mizuguchi on his decision, "At that time, I had two choices, basically. One was to adjust with the company, or the company adjusts to me, but the latter was impossible. So I decided to leave Sega. To me, I think this is a very healthy choice, because I get to do my own games now." Mizuguchi would go on to found Q Entertainment. He has expressed that he would be open to working with Sega again.