Sinistcha lives in cool, dark places such as behind shelves or under floorboards, and in the past it would sneak into houses posing as tea so that people would drink it. Despite the resemblance, it is not biologically related to Polteageist, and a Masterpiece Form is identified by a stamp of authenticity on its chawan's base. It wears a broken chasen, the bamboo whisk used to prepare matcha, on its head, and its eyes and body shape resemble Polteageist's.
Sinistcha debuted under Carmine's ownership in Roy is Crocalor, and Crocalor is Roy!.
Sinistcha's earliest card was a Rare in Twilight Masquerade illustrated by Kouki Saitou, the same set that introduced a Double Rare and an Ultra Rare Sinistcha ex by aky CG Works, plus a Special Illustration Rare ex by Saboteri. Pitch Black later added an Uncommon printing in the Mega Evolution era.
Sinistcha's earliest card, a Rare from Twilight Masquerade illustrated by Kouki Saitou.
A Double Rare Sinistcha ex from the same set, illustrated by aky CG Works.
A Special Illustration Rare Sinistcha ex, illustrated by Saboteri.
An Uncommon Sinistcha from Pitch Black, part of the Mega Evolution era.
It evolves from Poltchageist using either an Unremarkable Teacup or a Masterpiece Teacup.
Despite the resemblance, Bulbapedia notes they are not biologically related, though their names cross-reference each other's pre-evolutions.

ヤバソチャ
Grass · Ghost
Sinistcha is the Matcha Pokémon, a Grass/Ghost type evolved from Poltchageist, inhabiting a traditional Japanese tea bowl and wearing a broken bamboo whisk atop its head. Our catalog lists 5 Sinistcha cards across 2 sets, from Twilight Masquerade through the Mega Evolution era's Pitch Black.
PoltchageistSinistcha
No published index yet — too few of Sinistcha’s cards trade often enough to strike one.