Fujiwara-no-Mokou and Kaguya Houraisan (nee Fujiwara in this one) are characters from Touhou 8- Imperishable Night. To summarize a long and complicated backstory:

Mokou (and Kaguya in canon, but not yet demonstrated here) gained ressurective immortality from the Hourai Elixir. This power comes with the standard Touhou insane-lack-of-limitations. Also in canon, Mokou and Kaguya are "mortal" enemies, though Mokou has some severe tsundere issues regarding Kaguya, and the latter is aware of this and makes use of it.

"This is probably a stupid question, but . . ."