Dōsei (同棲, lit. Cohabitation) is a Japanese adult social simulation game developed by Tactics, a brand of the joint company Nexton, and first released on May 23, 1997, the same day as the famous To Heart by Leaf. The game was initially playable on the PC as a CD-ROM on the Windows 95 operating system, but was later re-released compatible for Windows 95/98 on September 14, 2000 under the title Dōsei Memorial Collection. The gameplay in Dōsei follows a plot line which offers pre-determined scenarios with courses of interaction, and focuses on the appeal of the sole female main character Manami Minase. The player assumes the role of the protagonist named Masaki Yamada who is living with Manami shortly after they have graduated from high school. Masaki earns money at a job, and when he returns home will have sex with Manami often; this process of work in the day, and sex at night repeats many times throughout gameplay. Four of the staff that created the game, Itaru Hinoue, Shinji Orito, Miracle Mikipon, and Shinory later became four of the founding members of the visual novel company Key.
Dōsei's gameplay requires little interaction from the player as most of the duration of the game is spent on simply reading the text that appears on the game screen which represents either dialogue between the various characters, or the inner thoughts of the protagonist. Every so often, the player will come to a "decision point" where he or she is given the chance to choose from options that are displayed on the screen, typically two to three at a time. The time between these decision points is variable and can occur anywhere from a minute to much longer. During these times, gameplay pauses until a choice is made that furthers the plot in a specific direction, depending on which choice the player makes.
The story begins with two main characters shortly after graduating high school who are living together. They are, the protagonist Masaki Yamada (山田 まさき Yamada Masaki ) who the player assumes, and the heroine named Manami Minase (皆瀬 まなみ Minase Manami ) who he is living with; both characters' names can be altered to anything the player chooses. There are two parameters related to Masaki and Manami which change depending on decisions the player makes during gameplay. Masaki's parameters are physical and emotional strength, and the Manami's parameters are affection and lewdness degrees. How much money the protagonist has from his job also is a factor in the game; the player starts out with 100,000 yen (about US$1,100).
The story opens on Sunday September 14, 1997. One of the main attractions during gameplay are the viewing of the hentai scenes depicting Masaki and Manami having sexual intercourse. Typically, Masaki will come home from work and after a short conversation with Manami will have sex with her. This continuously repeats many times day in and day out, causing much of the gameplay to occur during sex scenes. Depending on the choices, the protagonist may even become unfaithful to Manami which is when more sex scenes with five different women can be viewed. The player is occasionally given the opportunity to rest, but if he rests too much then he will soon go bankrupt and the game will end.
Dōsei was Tactics's first game. Planning was headed by YET11, who also did programming, and Don, who wrote the scenario, though this person did not contribute in future Tactics titles. Art direction and character design was headed by Itaru Hinoue, and was the second time she had ever contributed in a visual novel. Hinoue also contributed with the computer graphics along with Miracle Mikipon, Shinory, and Suō Akiyama, and Mikipon also worked on the game's animation sequences. The music in the game was mainly composed by Shinji Orito who had at the time moved to Tactics after working under Leaf for three games. Hinoue, Orito, Mikipon, and Shinory later became four of the founding members of the visual novel company Key founded in 1998. Three others—Myū, Paste, and Ishisan—also helped with the music, but between them only composed one third of the game's soundtrack. The game's soundtrack was released bundled with the soundtrack for Moon., the game Tactics made after Dōsei, and was called Dōsei and Moon. Original Soundtracks. The album contained a single CD and was released on August 10, 2000 at Comiket 58 by Exobitant Records. The disc contained thirty-one tracks; the first fifteen pertained to Dōsei and the latter sixteen were for Moon..
Dōsei was first released to the public in Japan on May 23, 1997 as a CD-ROM playable only a Windows 95 PC. The release date was coincidentally the same day Leaf released their visual novel To Heart. Nexton, the publishing company Tactics is under, re-released an updated version Dōsei compatible with Windows 95/98 on September 14, 2000 under the title Dōsei Memorial Collection.