Rider
Rider awaited Sakura's response with anticipation, wanting to go into the room so she could congratulate her master. However, before Sakura could reply, Rider heard her let out a adorable squeak of surprise. Normally, Rider would have been a little nervous at Sakura emitting such a sound, but she knew Shirou would never do anything to harm Sakura. Instead, she listened in with a smile on her face as Shirou asked her where they should go, referring to her as his "princess". Her smile broadened further as she heard Sakura's response.
Seems like Rin was right, she thought with amusement, before entering the room just in time to see Shirou carry Sakura out of the opposite door and towards the stairs, her master making some odd and slightly-worrying giggling sounds as he carried her away. Then, somewhat unexpectedly, she addressed Rider over their mental link.
Sure, you can come! she said, her voice full of joy.
A look of confusion came over Rider's face on hearing her reply.
Does she...? Really?
Not that Rider objected to the idea of a threesome with Shirou and Sakura. They were both attractive people who she liked and trusted, after all, and if it made Sakura happy she would be glad to go along with it. But it was certainly not something she expected her to offer, at least at this stage of her life.
I'm probably just misunderstanding her..., Rider thought, shaking her head to clear her mind.
Nevertheless, she approached the room somewhat cautiously, not wanting to intrude on something Sakura did not wish for her to. Approaching the door, she heard Shirou and Sakura in conversation, and decided to listen in for a moment, to make sure it was OK to enter.
"Well, of course, how could I not be? We're finally getting married, Shirou! I'm so happy I could burst", she heard her master say. "Can you imagine a more amazing day?"
Rider smiled once more, her heart warmed by the fact that Sakura was feeling such happiness. Happiness that she could never have dreamed she would see again even a day ago.
"I guess you're right, Sakura. Today's been pretty amazing", Shirou responded.
Figuring that it would be better to be polite, if only for Shirou's sake, Rider knocked on the door.
"Sakura, Shirou, can I come in?" she asked, her voice full of joy.