Pokémon: Yuyuko Saigyouji (Touhou Youyoumu ~ Perfect Cherry Blossom)
Type: Ghost/Grass
Ability: Grassy Surge/
Manipulation of Death (HA)
Signature Ability: Manipulation of Death- Every attack that hits has a 2% chance to instantly faint the target. Foes that make contact have a 1% chance of fainting immediately.
Moves: Ghastly Dream*, Shadow Ball, Energy Ball, Petal Blizzard, Shadow Sneak, Destiny Bond, Protect, Swagger, Flatter, Ingrain, Every OHKO move, Synthesis, Phantom Force, Attack Order, Defend Order, Heal Order, Crunch, Stockpile, Swallow, Spit Up, etc.
Signature Move: Ghastly Dream: Ghost-type, Base 90 Power, Special. Deals damage based on Yuyuko's higher attack stat. Has a 2% chance to instantly faint the target.
Signature Z-Move: Saigyouji Flawless Nirvana- Ghost-type, Status. Upgraded from Ghastly Dream via the Yuyukonium Z. Instantly faints the target. Affects Normal-type Pokemon.
Stats: 110 HP/110 Atk/110 Def/120 SpA/110 SpD/30 Spe (590 BST)
Tier Warning: Manipulation of Death, Ghastly Dream and Yuyukonium Z are banned in Overused under the OHKO Clause.
Reasoning: Yuyuko Saigyouji first appears as the Final Boss of Perfect Cherry Blossom. She's a ghost who shares the same power as the Saigyou Ayakashi, a youkai tree, and is mostly associated with Cherry Blossoms (hence the Ghost/Grass-typing). However, her Ability and Signature Move/Z-Move are a direct reference to her ability in Touhou canon: The ability to cause things to die. Since OHKO moves are banned in OU, this means that Yuyuko cannot use her ability in OU, either, as this turns ALL of her attacks into potential OHKO moves, albeit at a very low chance (Ghastly Dream, her signature has the highest non-100% rate which is 4% total). Her Z-Move is based on her Last Word from Imperishable Night, which is said to be the incarnation of her ability. Thus it only seemed fitting to make it a guaranteed OHKO and one of the main reasons to use Yuyuko in Ubers: If she can get a chance to fire her Z-Move off, that is one target immediately gone from the battle. Her other moves are standard ghost- and grass- fare, save for a few. Yuyuko's projectiles often take the form of spectral butterflies, hence why she also has access to Attack Order, Defend Order and Heal Order. Lastly, Yuyuko is known for her immense appetite, hence Stockpile, Swallow and Spit Up.
Playing against Yuyuko in Ubers is a risky affair because even if a 'counter' switches in, there's a chance, however small, that her ability might just pop up and ruin whatever your would-be Yuyuko counter would attempt by promptly saying 'WELCOME TO DIE!', before promptly, well, killing them. Remember,
people die if they are killed. That and a predicted switch could result in a Saigyouji Flawless Nirvana to the face and... yeah. They still die. In OU you don't have to worry about that, thankfully, since OHKO Clause prohibits the use of Ghastly Dream and Manipulation of Death, but Grassy Surge can be just as annoying, given Yuyuko's great bulk. She can prove to be a very annoying wall that's tough to break. After all,
you can't kill something that's already dead.
Her stats are a bit different though. In the games, Yuyuko is often depicted as slow-moving, but has wide range coverage and, in the fighting games, good options for any situation, so her stats save for Speed are all very high and well-rounded.