Sinistcha is a Pokémon made of matcha tea. As such, its main body is entirely green, except for its mouth and swirly eyes, which are yellow in color. Its arms are thin and its hands have three digits, and it has two small legs. It inhabits a 茶碗 chawan, a tea bowl used in the traditional Japanese tea ceremony. The chawan it inhabits is black and white, with a few cracks sealed with green all over it, and a swirling, face-like pattern on its side. Atop its head, Sinistcha wears a 茶筅 chasen (a bamboo whisk used to prepare and stir matcha tea), which is broken and missing a few of its teeth. Despite appearances, Sinistcha is not biologically related to Polteageist.
Sinistcha lives in cool, dark areas such as the back of a shelf or beneath a house's floorboards. It pretends to be tea in order to trick people into drinking it, so it can absorb their life force. However, this act is rarely successful. Sinistcha is generally active after sunset. In order to prepare itself for battle, Sinistcha builds up its energy by using the chasen on its head to stir itself.
Sinistcha has two separate forms: Unremarkable Form and Masterpiece Form. Its form is determined by the form of Poltchageist it evolves from and cannot be changed.
The Masterpiece Form is rarer and has a stamp of authenticity hidden on the underside of its base. The only functional difference between the two forms is the Evolution item used by Poltchageist to evolve into Sinistcha: Counterfeit Form Poltchageist requires an Unremarkable Teacup to evolve into Unremarkable Form Sinistcha, while Artisan Form Poltchageist requires a Masterpiece Teacup to evolve into Masterpiece Form Sinistcha.
Masterpiece Form Sinistcha does not obey typical form inheritance rules when breeding, and therefore cannot produce Eggs that contain Artisan Form Poltchageist. All Poltchageist that hatch from Eggs will always be Counterfeit Form, regardless of the parent's form.
Despite being an apparent counterpart of Polteageist, Sinistcha's name resembles that of Polteageist's pre-evolution, Sinistea. The reverse is also true with its pre-evolution, Poltchageist. They are the only ecologically similar Pokémon with this distinction.
Additionally, Sinistcha and Polteageist have similar evolution methods, as they both require being exposed to a specific item to evolve into a specific form of their evolved counterparts.
Sinistcha and its pre-evolution are the only ecologically similar Pokémon to have multiple forms.
Sinistcha appears to be based on matcha tea that has been prepared into a drink in the traditional Japanese tea ceremony. Its design incorporates several utensils used in the aforementioned ceremony, namely the 茶碗 chawan (tea bowl) and 茶筅 chasen (tea whisk). It may also be based on a tsukumogami, being an inanimate household object come to life — in this case, equipment used in a Japanese tea ceremony.
Sinistcha's overall design resembles Polteageist, having similar eyes and a similar body. This may be a reference to convergent evolution or parallel evolution, which can lead to separate organisms independently developing similar or identical characteristics and body plans.
Sinistcha's forms may be based on stoneware forgeries. One historical way to combat stoneware forgeries was to create a stamp of authenticity on products, hence the stamp under Masterpiece Form Sinistcha. It may also be a reference to the difference between unremarkable household objects and rare variants of them which are seen as antiques or masterpieces. Another origin for Sinistcha's forms may be the different grades of matcha, or matcha imitations.
Sinistcha may be a combination of sinister and 抹茶 matcha, following the same naming pattern as Sinistea.
Yabasocha may be a combination of やばい yabai (dangerous; terrible), ヤバチャ Yabacha (Sinistea), and 粗茶 socha (low grade tea, coarse tea).