Fancy looking wings there, especially at the bottom. They're like Daiyousei's.
...and Yuuka has green hair and Daiyousei thought that she had experienced something similar to when Yuuka pat her head earlier too. Mom? Sister? Just an old friend?
The Seihou reference, for those that are wondering (as I was), is that Yuuka appeared in the Seihou game 'Kioh Gyoku'. During her spellcards in that game, she would display 4 or 6 wings.
Also, from her official profile for the game:
After Mystic Square's extra ending [...] Yuuka received the ultimate magic. The author himself doesn't know exactly what it is.
So I posted this way back on the comic it applied to, but I'll say it here too: The author explicitly refers to the part of the fight from the beginning to when Rumia first shows up with her cloud of darkness as "the first half."
Thinking about it, a fairy is basically an embodiment of nature. which normally is potentially much more powerful than most gods.
Following this logic the fairies of gensokyo have a huge potential that they rarely ever tap into, it wouldn't be much of a stretch to say that all fairies could amass the same level of power of Yuka under the right conditions.
The only loose end being the conditions themselves.