It seems like Kaguya has a major issue with the idea that Mokou is happier than she is. I hope she doesn't attack Mystia just to get to Mokou, I like this ship too much.

I wonder what it is in Eirin's mind?
Look at her face. She's pondering about something.
Could it be that she's a little depressed about the irony of Mokou being happier than Kaguya in this affair? Or perhaps she knows that if Mokou finds happiness with someone other than Kaguya, that Kaguya will never be truly happy? Or perhaps it could just be her being an obedient servant and not thinking about it at all. I can't wait until the next release.

Mokou said:

I wonder what it is in Eirin's mind?

Doylesian response: She's thinking, "Oh great, Kaguya's the villain again in this doujin." Kaguya's basically doing a "Love is for the weak!" monologue, here.

I'm honestly a little surprised Kaguya's not the villain more often, since the "Lunatic Princess" is custom-made for being a villainous noblewoman archetype. (I guess comic relief Teruyo NEET is just more popular just because not as many people do serious Touhou works as comedies. There's also the fact that people seem to like Reisen more, so like Remi being practically Sakuya's comic sidekick, rather than a respected master, Teruyo gets the side character role to the Inabas.)

Watsonian response: She's thinking that Kaguya's an immature brat for looking down on someone for being in love, but is too dutiful a servant to say it to her face.

NWSiaCB said:

Doylesian response: She's thinking, "Oh great, Kaguya's the villain again in this doujin." Kaguya's basically doing a "Love is for the weak!" monologue, here.

I'm honestly a little surprised Kaguya's not the villain more often, since the "Lunatic Princess" is custom-made for being a villainous noblewoman archetype. (I guess comic relief Teruyo NEET is just more popular just because not as many people do serious Touhou works as comedies. There's also the fact that people seem to like Reisen more, so like Remi being practically Sakuya's comic sidekick, rather than a respected master, Teruyo gets the side character role to the Inabas.)

Watsonian response: She's thinking that Kaguya's an immature brat for looking down on someone for being in love, but is too dutiful a servant to say it to her face.

Being the villainous noblewoman archetype also simply does not fit her canon personality at all; the only person she acts in such a way toward is Mokou, simply to keep the conflict (and thus her entertainment) going.