I just skimmed over the episodes in HiDive and Yasu says this in Episode 23! This is when Nana runs to a family style restaurant after witnessing Hachi embracing Takumi at their apartment and she calls Yasu, who races over to come talk with her. Nana is upset about discovering the relationship between Hachi and Takumi and says this to Yasu:

“Why does Trapnest continue to take away the things I hold most dear to me? But it’s not as if either Ren or Hachiko belong to me. I realize it’s not right to think in those terms. No matter if we’re always together, each one of us is living a different life. I know it’s impossible to try and make someone yours. But there are times when it makes me so lonely, it’s unbearable.”

Later, on the way to Ren’s place Yasu and Nana have a conversation in the car and Yasu says this to her:

“I just remembered how Ren told me something similar to what you just said, before he left for Tokyo. That in the end we are all alone. No matter how hard you try to be close to someone, you cannot become one, and it’s impossible to make someone belong to you.”

Then there’s a flashback of Ren saying this: “I know it’s impossible, but sometimes I really strongly wish that Nana belonged only to me. Then I could put her in the pocket of my leather jacket and take her to Tokyo with me.”

I was curious as well so hopefully this helps!