While it may be anti-climatic, I think the coated figure might be apprentice Xehanort. Given that his abduction of Kairi seems key to this story and put everyone on a collision course to be targeted years later on Destiny Islands by his heartless, I can't think of anyone else she would hate more.
She says the figure is either a memory or a phantom, so she seems pretty sure it's no one still alive. While some might think it would be too obvious, they did something similar with Vexen in the "Final Battle" trailer for KH3.
Perhaps Xehanort tampered with Kairi's memory to make her forget everything from her life on Radiant Garden as part of a final plan: maybe he didn't want her to ever realize her true power, which memory can have an effect over. The stars she sees in the final world may be representative of her friends and family from Radiant Garden, all gone because of what Xehanort did.
She might have more reason than anyone to hate this particular form of Xehanort, even more so than Ansem the Wise.