She is known for the memorial "First to Use the Spell Card System™" in the history of shooting games.
She is also the first official Windows boss of the Touhou series.
Her ability, "manipulation of darkness," sounds quite strong, yet she is only a poor stage 1 boss. It's kind of ZUN's joke.
She was intended to have bullets that mimic darkness, but it was not implemented. It later showed up in Imperishable Night.
The phrase "人類は十進法を採用しました" (Mankind adopted the decimal system) appears in Marisa's scenario of EoSD. It comes from Hiroshi Mori's novel "誰もいなくなった ～Thirty Little Indians" (Then There Were None ~ Thirty Little Indians) from "まどろみ消去 ～Missing under the Mistletoe" (Slumber Erasing ~ Missing under the Mistletoe).
Rumia doesn't react well at all to sunlight and dislikes the heat of summer, taking shelter from both in her own personal field of darkness. However, even she is unable to see in the dark and bumps into things on a regular basis while wandering with no goal in mind.
Rumia's non-spell card attacks and her spell card Moonlight Ray are similar to the attacks of Mystic Square's first stage boss, Sara.
There is a municipality and a town Rumia in north-western Poland, that has been established in 13 century and earned a town rights in 1954. ZUN probably wasn't aware of it when he created the Youkai of Darkness.
If it weren't for Rumia's amulet, she would have been one of the few people that don't wear any headgear.