FMA: We promise.

at the beginning of the story Edward and Felisha weren't fond of each other until Edward confront Felisha about her alchemy as she perform it without a circle. when the jig was up Felisha admitted that she too break the taboo. She and her sisters perform to bring their parents back, but the end result was not it was suppose to be. not only Felisha loses her right arm but also her sisters life.
After hearing her story Edward decided he should not hate her as they both suffered the same fate and tried to made up their partnership plus promising Felisha that she will get her arm back. so this where Felisha realized she has fallen in love with her partner.