The thing about Arima is that he’s very much the child soldier all grown up. It’s heavily implied he has something to do with the garden, either coming from it or being its forerunner. He has had no life outside of the CCG. And I’m willing to bet that he’s been part of V for a very long time as well.
I’m honestly, and maybe mistakenly still kind of rooting for him, but not to turn out to secretly be a hero - rather to make a choice that saves Kaneki or that ultimately saves everyone.
Fura calls Arima a “yes man” and I think we’ll be coming back to that. I think for his entire life, Arima has been ordered around by these large powers and performed perfectly for them. Their perfect little soldier, their ideal puppet and tool.
Arima is hyped up as a destroyer and a power to be feared, but in a way, he’s sort of ironically without any agency of his own.
The struggle for agency is actually a pretty central one for Kaneki, and thematically in both Tokyo Ghoul and Tokyo Ghoul RE.
The struggle for self definition despite how the world and greater powers would define you.
To play your own role despite the role others would have you play.
It is, in a way, the central struggle of tragedy - the struggle against Fate and the Inevitable.
I think that Arima’s time will come when he finally makes a choice of his own, maybe to stay his hand when he is ordered to kill, or maybe to kill when he is ordered to stay his hand. Since his character is so wrapped up in death, those two possibilities seem most likely. But I think Arima will get one shining moment to say “no,” that will earn him that place on the Joker card.
In this way, he is a clear contrast to Eto, who is defined by agency and defiance - by charting her own course and defying the rules of others.
pictured above: agency; charting one’s own course; defying the rules of others; also, eyeball licking.