Selene Eiselstein (セレネ・エイゼルシュタイン Serene Eizerusyutain ) is the true daughter of Professor Eiselstein, and is no longer of this world. Edward and Alphonse, when they had visited the Professor with their teacher in the past, hadn't gotten to see Selene before they had left. When Edward recalls their first meeting, during a train trip with Alphonse and Major Armstrong , he believes that Selene was cold-hearted, mainly because she wasn't present when Izumi took him and Alphonse back to Dublith. Alphonse, unsure of the reason, starts to suggest that Selene might have been sick, when the Major suggests that maybe the young girl found it difficult to say goodbye to the brothers. Selene was always quiet, yet very intelligent and an alchemic prodigy.
Selene used to help her father with his alchemic experiments as he tried to obtain the legendary amplifier: the Philosopher's Catalyst. However, she got caught in a rebound from one of his attempts to create a prototype of the Catalyst, and her body mutated. To save her life, Wilhelm tried to use the prototype Catalyst, and while it did that, Selene continued to change, and became the girl known as Armony. Armony however, having memories of playing with Selene as a child, has no knowledge of the fact that she (in a sense) is Selene, and believes that Selene was her older sister who had died. There is a grave for Selene at the church near Hiessgart that Armony often visits and leaves behind an Etherflower for her, but Armony does not know that the grave holds no body of her "sister".
When Wilhelm confesses what actually happened to Edward and Alphonse, he admits worrying about how quiet Selene had always been. He explains that, initially, his continued experiments with developing the true Philosopher's Catalyst were due to his desire to bring back Selene. In time, however, he realized that that would be impossible, and more than that, he had come to love Armony as his true daughter.