If you take a look at the Fire Emblem Wiki and search "Grima", you will know that Grima was an evil dragon who the Grimleal were desperate to ressurect. In the future that the first version of Lucina comes from, Validar casts a spell on Robin that allows Grima to take possesion of his/her body. So in a way, Grima is Robin and Robin is Grima.
In the story, Grima lives in the form of a spirit able to amplify the power of the one he possesses. His goal is to create dark clones of multiple fighters, possess the strongest fighters. and cast the trophies of them into the rift, allowing Red Gai to feast on their souls, but there is a way for those in the rift to escape.
i know my wifi is getting fixed tomorrow so i cant wacth Youtube or listen to any music and i can kinda go on wiki but i have things to so i cant really also i just i am taking a break from finish the story