This sub-article lists unofficial content associated to characters or other objects that originally appear in the Seihou Project series.
The Seihou Project has a rather decent fanbase, but compared to the Touhou Project, it isn't as well-known. On a general basis, fans of Seihou are very likely to also be fans of Touhou, mostly due to ZUN having a strong contribution to its first two games Shuusou Gyoku and Kioh Gyoku. Due to this, Seihou appearing in derivative works and fanart related to Touhou often occurs in itself, regardless of the two settings. Also, western fans who strongly advocate Seihou often say that they're sister-works with Touhou and that they often refer to Seihou as "the sister of Touhou". With derivative Seihou works, there's currently no sources of actual anime sold, hardly any fangames and rarely known doujin music, although there's one known CD called Alternative that's a rare exception to been a Seihou-only CD arrangement, as well as somewhat common CD arrangements that contains both Touhou and Seihou.
Fans do make note that the gameplay on the Seihou games are much harder than Touhou, having fewer, but faster bullets on the screen. However, it's very rare for fans to make note if a new Seihou game would be announces. Similar to Touhou's scheme of using 東方ＯＯＯ, fans will sometimes name the Seihou games this way to prevent confusion. i.e. 西方秋霜玉, 西方稀翁玉 and 西方幡紫竜.
Banshiryuu, the third instalment of the series, is sometimes viewed in a negative light by some, classifying it as more of a fangame with no importance compared to the previous instalments. There are various reasons for this: VIVIT-r isn't the original VIVIT, ZUN had no involvement, the music, or it's characters that débuted in this game.
Bare in mind that VIVIT and VIVIT-r are different robots and Vivit isn't, been a daughter of Erich. Most of this paragraph is ambiguous whether this refers to both androids, and strangely on some occasions Vivit; this is because it's quite common to mix up the two as the same android, along with mixing up Vivit. Anyway, based on the fanbase associated to the Seihou Project, VIVIT is one of the most popular characters in the fanbase. She may appear to be an icon for the Seihou Project. With the Touhou Project however, her popularity can be said to be the average of a typical PC-98 character, where even the fans of Touhou may put her on their the top ten favourite list. In relation to this, it's common to see VIVIT appear with characters from the Touhou Project in fanon; this is probably due to officially visiting the Hakurei Shrine in Shuusou Gyoku's Extra Stage, mostly in relation to Reimu Hakurei, where the comparison here is their protagonistic roles to their respective projects. It's also common for VIVIT to appear in Touhou doujin games along with Erich. The most notable ones are Touhou Soccer Moushuuden and Touhoumon, which also has Vivit in the latter.
- Japanese fans sometimes call VIVIT "びびっと" (Bibitto), which is a translation of "VIVIT".
- A cactus is usually associated with VIVIT due to been powered by cactus energy, which is produced from cacti.
- Despite being a Japanese word, it's common for Westerners to call the cactus energy as "Saboten energy", keeping it romanised rather than having a full translation.
- Ambiguous whether this also refers to VIVIT, VIVIT-r has occasionally been depicted with Hirano Sakurasaki due to both been heroines.
- Just like in her official appearance, Vivit is commonly drawn nude (but with her body commonly indescript).
- Mei and Mai's most notable line among fanon is "Mei:But, Mei and... Mai:Mai are... Both:Invincible!" (Mei:でも、めいと… Mai:まいは… Both:無敵だよっ！).
Perhaps due to his name, or perhaps due to his appearance or personality, among Seihou Project characters, Gates is an unusually popular character for some reason, and in the "Second Touhou Series Popularity Poll", and in the "Third Touhou Series Popularity Poll", among Seihou characters, he was the most voted-for character. (In the first one, he wasn't nominated, and in the fourth one and thereafter, it's not possible to vote for Seihou characters).
- He is sometimes compared to Rinnosuke Morichika, stating Gates as the "Kourin of Seihou".
- There's a lot of fanart depicting Gates as a rather serious type of solider.
- Erich is notorious for his use of homing lasers by fans.
- He's one of the few Seihou Project characters to appeared in a Touhou Project doujin game along with VIVIT. It's only known among the western fanbase so far that he has appeared in one fangame, which is Touhou Soccer Moushuuden.
- Because of his "Fundoshi Barrier" in this game, some fans have jokingly stated that he, along with Rinnosuke Morichika, is gay.
- Although Morgan's shown with a long ponytail in her official art, a lot of fans still draw her without one.
- Dues to similarities, fans speculate that Sakuya Izayoi was originally based on Muse.
- Some fans will occasionally get confused if it's either Hirano or Reimu depicted in an illustration.
- Ambiguous whether this also refers to VIVIT, but Hirano has occasionally been compared to VIVIT-r due to both been heroines.
- Because Felia was seen saying "I'm the strongest" and that she has short blue hair, some fans speculate that Felia was based on Cirno. Also, both Cirno and Felia have childish personalities.
Like other Banshiryuu characters, compared to the Seihou Project, or even the Touhou Project, Icarus is a very rare character to appear in both the Eastern and Western fanbase. Very little fanart of her exist and it's mysterious if any fangames has her in one; it wouldn't be the fact that she's an unpopular character that has a negative view, but rather is more towards being considered a "legendary" character amongst the fanbase as fans do indeed talk about her.
- Her phrase "I'm the last boss, I'm strong!" (ラスボスだよ、つよいよ！) is commonly associated with her in the fanbase.