Yuki expresses deep resentment toward Tengen for the sacrificial nature of the merging process. She confirms that while Riko Amanai was the vessel twelve years ago, there were others before her.

Jujutsu Kaisen Chapter 202: "Blood and Oil" Recap & Analysis

Tengen admits that because the merger with Riko failed, they have begun to evolve beyond humanity. Yuki is disgusted that Tengen "accepted" this outcome, effectively rendering the deaths of past vessels meaningless in her eyes. Kenjaku’s True Master Plan