Rider
Rider continued to listen to Archer and Rin's conversation, smiling a little as her master excused herself and ran back upstairs to bed, suspecting that Shirou would very soon be feeling anything but "sad". Meanwhile, though, the discussion between Rin and Archer rapidly became far darker and more serious than she would have expected, with Archer admitting that, in his Grail War, Caster had somehow killed his version of Rin. Rider was rather surprised by the revelation, but not as much as Rin, who was clearly unnerved.
Then, Rin made a comment that was typically lacking in tact and compassion, causing Rider to have to restrain herself from audibly groaning. Before Archer could respond, however, she heard her master's voice come over their mental link.
Rider! Come on, we're going out, Shirou needs new clothes! she said, her tone of voice indicating that she was in a very good mood.
Eh? Why do I need to go if Shirou needs clothes? she thought, somewhat confused.
Still despite her confusion, as Archer responded to Rin in approximately the manner she had expected before storming out of the room in anger, she decided that she would much rather go out for a nice walk with her master than stay around here, in this uncomfortable situation. So, she quickly replied to her master.
OK, I'll be upstairs shortly, she said.
As she did so, though, she heard Rin speak once more, seemingly to herself.
"It must have been really bad to get that sort of reaction from him", she said, seemingly not understanding her mistake. "Geeze, I didn't know Archer could be so touchy."
With that, Rin turned to follow Archer out of the room. As she did so, Rider turned towards her, a frown on her face, and spoke.
"Rin, your response was somewhat... lacking in tact", she said. "Anyway, Sakura wants me to go out shopping with her and Shirou, so we'll be out for a bit."
With that, she watched as Rin ran out of the room, before turning to head up the stairs to Sakura's room, dressed in her normal casual outfit and glasses. Arriving at the door, she knocked on it politely.
"Sakura, Shirou, I'm ready when you are", she said, not bothering to question why her master wanted her to come in case it might ruin her good mood.
Besides, it would probably be fun, and it would be nice to have a chance to get to know her new master and the new Shirou a bit better.