His love and care for us pass all the tests
The five names are five pillars of DH. It is not unusual if a pillar is suddenly replaced. After all, DH is only a small part of something bigger. As one goes deeper into the dark world, there's no end how far one can go.
The five of them know about each other. They know each other's secrets. Knowing more means that death will seek you faster if you betray. Trust is a double-edged sword in the inner circle of DH. They trust each other's abilities, but they won't hesitate to remove the betrayer when the time comes.
Rie already knows this. Yuta is still young. He doesn't know much about DH.
"Rie is lost in her thoughts again." Nina laughs. "Sending Yuta away has taken away all the life from her. I don't think that she can do such a difficult job if she's always in the land of thoughts."
"Why don't you just worry about yourself?" Rie looks at her. "You were nearly caught last time."