Rider
Rider smiled at Shirou's display of affection, a warm feeling running through her at what was unfolding in front of her.
"Guess I need to find a ring, huna?" Shirou said, addressing Sakura.
I'm fairly sure you don't..., thought Rider, having guessed at the contents of Sakura's bag.
Sure enough, Sakura pulled out a small box from the bag she had been guarding like her own child.
“Nope! I b e a t you!” she said, giggling somewhat creepily.
“Now do it right!” she commanded.
Rider, meanwhile, simply watched with a broad smile on her face, wanting to see what would be one of the happiest moments of Sakura's life. For a moment, Shirou acted uncertain, and a frown came across Rider's face until she realised he was surely just playing around. Regardless, as soon as he opened the box, he seemed far happier about the choice.
"Tohsaka picked this out, didn't she?" he said, before dropping down on one knee.
Makes sense. Rin is a gem expert. I hope she's not intending to blow this one up, though..., Rider thought, a big smile on her face.
Then, Rider watched intently as Shirou offered the ring up to Sakura, asking her to marry him. Sakura, of course, instantly accepted with a bright smile on her face, one which was matched on the face of her loyal servant. Tears of happiness formed in Rider's eyes as she watched Shirou slip the ring onto Sakura's finger and lean in for a kiss. Rider continued to watch, pure joy in her heart, as Sakura admired the ring that was now hers, before pulling Shirou down onto the bed, gripping him tightly.
"Congratulations", she said with a bright smile on her face, her voice almost shaking from the happiness she felt. "You two will be very happy together."
Rider remained there watching, drinking in the happy sight, her heart still full of joy, waiting for some indication, verbal or by action, that she should leave. Just being here to witness this was worth everything she had ever done to get to this point. She would do anything, literally anything to ensure that Sakura kept this happiness.