The boy between Xigbar/Braig and Xemnas/Xehanort is Salim Bradley, a Homunculus who was named Pride in the manga of Fullmetal Alchemist. He poses as the adopted son of Fuhrer King Bradley, who is also a Homunculus. The guy that's between Terra and Vanitas is an OC named Crimson that I made up to be the King of the Heartless/Golems from Fullmetal Kingdom, The Fanfic. And the guy after Vanitas is Ling Yao, from FMA: Brotherhood, right now, he's got the Homunculus Greed inside him.