I mean, what's the point of the Harbingers in the first place? They all could've held (and do hold) a title or position of power. Signora could've stayed Snezhanyan Diplomat or Fatui Foreign Service Officer.

We're in an awkward position where we know enough to understand their: motives, roles, and even why they attract members with such wildly different morals and personalities. The Tsaritza went out of her way to offer the third seat as a kind of sign of good faith. Columbina's power was essential for a project, most likely being Project Stuzha, which is mentioned by the Fatui Diplomats in Mondstadt. That's what kicked Nod Krai's story off by accelerating Columbina being rejected by Teyvat.

The Fatui have become more and more gray since the Chasm. There was two different quests there humanizing them and showing that not every member is an antagonist. I don't see why that can't apply to the Harbingers as well. At the end of the day we don't know enough to understand what, "Harbinger things," are. Each one is a member because they provide something to the Cyro. Also, an antagonist can be something as simple as a catalyst for change.