She swapped bodies with Mai due to her , but swapped back after realizing that she did not have to be exactly like Mai.
Grace, and the audience, finds out the truth: she and her kids are actually the ghosts haunting the mansion, because years earlier, she had a breakdown and murdered her children before committing suicide.
By mere coincidence, Sayu happens to be sitting on a bench at a nearby park where Mishima would also usually sit as she waits for the train to arrive.
Sayu and Asami decide to hang out after work and while walking she tells Asami about her past.
Finally, she got so frustrated that she blasted the entire contraption in her rage.
As Korra's uncle, Unalaq was her spiritual mentor.