Artificial Girl

Artificial Girl is a series of three H-games (and an expansion pack) made by the Japanese gaming company Illusion. It allows the player to design from scratch an ideal girl, picking hair color, eye color, voice, and bust size. From 3 onward, one could also choose certain personality traits to add. Having created said character, the game allows you to walk around in a visually unimpressive virtual world and, well, interact with the girl that was just created.

After creating the girl and entering the world, the only thing that can be changed about the girl (barring anything done through mods) is the clothing she wears, though the available options are quite in depth.

Obviously never gonna see an international release, as is Illusion's policy.

Tropes used in Artificial Girl include:

Brother-Sister Incest: Not only can you have the girl(s) address you as Onii-chan in 3, but also give them a "likes family" trait.
Character Customization: Arguably the purpose of the game. Interesting in that the player character is not the one that allows the most (or any) customization. But you probably expected that. Also of note, the most recent clothing editor is actually very in depth, allowing one go so far as importing custom textures to apply to a number of base shapes.
Game Mod: There is a thriving modding community who have created an absolutely huge library of additional content, including props, clothing, replacement skin textures for the girls and even mods to the girls' 3d models themselves.
Harem Seeker: Artificial Girl 3 and the expansion allow you to have up to five girls in a single savefile.
Jiggle Physics
Les Yay: Actually possible in 3, with the "Likes Girls" trait.
No Export for You: Illusion has a strict policy of not allowing their games to be exported.
Video Game Cruelty Potential: Of sorts. At least in 3, when the player character is nearing climax (the "mushroom" is red with veins popping out), girls will frequently ask if the player is close to cumming. If a yes response is given, they'll either tell the player it's okay to cum inside or to pull out (in some ways, depending on personalities). Whether or not players will heed the request is entirely up to them; doing so will generally prompt a remark of some sort.