[Kirayuki]

His love to Ren pretty much sum up his obsessiveness.

Spoiler for Observer:

And Observer will always take action whenever the Throne is prone to destruction either by something like Hajun or the technological level of the current world managed to catch up with Throne's technology (in other words, capable of removing/destroying it).

That's why they are mostly spotted when the current reign of the Throne is about to end.