“EX(Extra) Rumia” (EXルーミア, Ekusutora Rūmia) is an old Touhou Project fan theory suggesting that Rumia has a sealed extra boss form. It relies on how the ofuda ribbon on her head is a complete mystery and how she's unable to remove it by herself.

We are first told that Rumia's ribbon is an ofuda that she can't remove in Touhou Koumakan's “omake.file”, then left with no answers for an entire decade. It was finally revisited in Volume 3 of Strange Creators of Outer World which adds that nobody knows who put it on her, then her Dai Touhouten¹⁾ plaque mentions that it was supposed to contain secrets within but has yet to be brought up.

She has no official appearance since it's just a fan theory, so people are free to speculate. The ribbon may float, become a halo, or outright disappear. Her hair could stay short or get longer. As for her accessories, she may get jet black wings, a large black cross or sword, etc. Most speculate that her sealed form is mature and/or adult-like, but some keep her body the same.

• It has yet to be discovered when people began to speculate about the existence of EX Rumia.
  • Touhou Koumakan released on August 11, 2002 and the theory has been mentioned in the first UFO Rumia-tan thread in 2005,²⁾ but it's implied that people were well aware of the theory.
• The large black sword could be a stormbringer, but it doesn't mean that it is one.
• A number of copycat theories for early Windows Era bosses (e.g. EX Letty, EX Cirno, EX Wriggle, etc.) have emerged, but they aren't as put together or have no basis in canon.

• The Legend of Twilight - As a secret final boss.
• Touhou Youreimu (“Osana Reimu”)

• EX Rumia on the Niconico Encyclopedia
• EX Rumia on the Pixiv Encyclopedia