Fujiwara no Mokou
The look Mokou gave Sanae was one of near confusion. How what? Nothing she had done was particularly unusual, was it? Sure, using magical abilities for actual damage rather than playing by the rules wasn't allowed any more, but even a relative newcomer to Gensokyo ought to know that it was a polite agreement rather than an actual limitation. Everything else was just--
Ah, right. She'd been so used to everyone who actually came after her knowing about the immortality (or being so focused on fighting for other reasons) that it had completely slipped her mind that someone might not know. In hindsight, a flashy self-immolation to clean up some injury probably looked a lot worse than it was--
Okay, it looked exactly as bad as it was, that dull ache everywhere. Still vastly preferable to having her arm rot off entirely or being eaten by a centipede, of course.
"It's 'cause I'm immortal."
That should cover it.