“Ultimate Sadistic Creature” (アルティメットサディスティッククリーチャー), often shortened to “USC”,¹⁾ is the nickname of an extremely sadistic fan interpretation of Yuuka Kazami from Touhou Project, which was presumably borrowed from a fan comic on May 14, 2008.²⁾

The term originates from a Touhou Project fan comic, Touhou Acrobatica (東方アクロバティカ), which was illustrated by “Ore to Umi” (俺と海). Back on May 14, 2008, we see Kanako slap away Cirno's offering of frozen frog dango,³⁾ which builds up to Yuuka listening to Cirno's sob story and glaring at Kanako, as Marisa comments that the “Ultimate Sadistic Creature Yuukarin” had taken an interest in her.⁴⁾

It isn't clear why these exact words were chosen to name the concept, but some speculate that it stems from the “Ultimate Battle Creature” in Bio Booster Armor Guyver. As for the concept's popularity, it had likely gained momentum with the release of Touhou Hisouten on May 25, 2008 as fans began exploring the idea of Masochistic Tenshi, then people started pairing them up.⁵⁾

The simple explanation is that Yuuka is consistently described as a “very strong youkai” and fans turned her playful teasing into bullying. Despite this being a weak proposal, some fans swear by the concept, typically dragging in her PC-98 past of verbally abusing characters to provoke fights. Of course, this just veers us into the unresolved issue of whether the PC-98 games are canonical to Touhou Project.

• A proper Japanese rendering would be “Kyūkyoku Kagyaku Seibutsu” (究極加虐生物).
• The influence of Ultimate Sadistic Creature has become fodder for Yuuka/Wriggle and Yuuka/Tenshi, which are both terrible ships involving characters that have never met in canon.

• Masochistic Tenshi - Point of origin. Arguably caused this fan interpretation.
• Sadistic Sanae

• Ultimate Sadistic Creature on Pixiv Encyclopedia
• Ultimate Sadistic Creature on Niconico Encyclopedia