Mossador

"I do not know the place from where old Ser comes. Things maybe are different there, I hope. But here, in Meereen, before Daenerys Stormborn, they own us. So we learn much about them, or we do not live long. They teach me what they are. "Mercy", "fair trial", these mean nothing to them! All they understand is blood!"

- Mossador

Mossador is a recurring character in the fourth and fifth seasons. He is played by guest star Reece Noi and first appears in "Oathkeeper".

Season 4
He attempts to convince his fellow slaves to join Daenerys Targaryen and defeat the Great Masters, though he meets resistance from skeptical older slaves who have lived through failed slave rebellions. After Grey Worm motivates them even further than Mossador and provides them with weapons, they launch a revolt against the Masters.

Season 5
Mossador, along Grey Worm, report to Daenerys Targaryen that White Rat, one of the Unsullied, was found with his throat slit at a brothel, murdered by the Sons of the Harpy.

Mossador, apparently a member of Daenerys's Small Council, is present when the Council debates what should be done with a captured Harpy's Son. When the queen decides to give the prisoner a fair trial, Mossador interprets Daenerys's decision on the matter to mean that her hands are tied, and without permission, kills the captive. Daenerys, has him brought out in chains before a crowd for execution, as a warning to everyone, freed slave or Great Master, not to pervert the course of justice. He pleads for forgiveness but she has Daario cut off his head, causing a riot.

Behind the scenes
The character was originally credited as "slave" in Season 4. Season 5 casting news revealed the character's name as "Keyr", but on March 21st it was revealed the character's name was changed to "Mossador".

In the books
In the A Song of Ice and Fire novels, Mossador is the name of one of the three brothers of Missandei who were enslaved alongside her and made into Unsullied and one of two who survived the training.

So far there is no indication if the TV character is meant to be Missandei's brother, or if the TV writers just re-used a name from the novels for a separate character. The latter is much more likely, since Mossador and Missandei had no real interaction in the character's brief tenure.