In Version 1, it is clear that it was Ava. By the time Gula revealed this to the Player, the other Foretellers had already come to accept that war was inevitable.

Who is the traitor in KHUX?

Luxu explains his mission, and claims that it is impossible to avoid the Keyblade War, and that he will set off after this world ends. Assuming that the Master has intentions to destroy this world, Ava accuses him of being the traitor responsible for sowing distrust between the unions, and he brandishes his Keyblade.

Who killed Strelitzia Kingdom Hearts?

In the latest Kingdom Hearts χ update, it is revealed that Darkness killed Strelitzia while possessing the body of Ventus. As Darkness steps into the light, it assumes the form of Master Ava, showing that Ventus believed he was merely having a meeting with the creator of the Dandelions.