Those feelings were the ones Ru told her to treasure. Even though he asked her out so she could forget, Sumire understood that Ru would never directly interfere if Yuhi returned. She thought she forgot those feelings with the pain of losing Ru, but it turns out that is not the case here.
"Hold on." Arashi stopped her. "I feel like I already know what you're going to say, and that is fine."
"It is?" She raised her eyebrow.
"No it isn't, but--"
Sumire laughed when she saw him panic. "Alright, I understand it I won't immediately say it. Do you think I am that cruel?"
"I think you are the cruelest person I know, but if I say anymore, I fear for my life."
A memory from a long time ago came back to her, and she paused. She said something similar to Yuhi in the past, and he questioned why she said it.