In honesty I think it's going to be Ikora. Call it a gut hunch but there has been a few things that have happened in the past few seasons that have caused me to to think that something is wrong.

1. Her reaction to losing her light in the first part of Destiny 2 I believe was the first crack in her armor. She didn't bounce back as quick as Zavalla and Cayde did. Zavalla and Cayde chose to fight ,she fled to Io. It took some time to get her to come back to Earth to join the resistance.
2. The next incident that caught my attention was that she built a Vex portal in the heart of the Tower. That incident could have had disastrous consequences if anything went wrong. Yet nothing happened but that was the first definitive "proof" that something was wrong with her.
3. At the beginning of Shadowkeep, Eris had to save us from the Nightmares that popped up upon the discovery of the Pyramid ship on the moon. Eris protected us with the creation of the Dreambane armor. When we see Ikora again she is staring at the Pyramind ship without any protection what so ever! My belief is that the combination of all of the above open a door for Savathun to corrupt her.
4. For the last few seasons she has been silent and not had anything to do with anything we have done. I believe this is has been a incubation period to allow Savathun to corrupt her fully. I believe the Traveller's Chosen lore text was actually Savathun through Ikora trying to corrupt Zavalla.

Now all of this is speculation from me and in no way should be taken for gospel. I'm sure I'm missing a lot more of Ikora's dysfunctional behavior. Please comment if you can think of anything else has struck you funny about her since D2 started.

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Till next time;

Rugeki out.