Ysmir is the Nordic aspect of Talos (Tiber Septim). He withstood the power of the Greybeards' voices long enough to hear their prophecy. Later, many Nords could not look on him without seeing a dragon. Ysmir is a name that has also been attributed to Wulfharth and Pelinal Whitestrake, although the authenticity of these stories is unclear.
According to the Greybeards, the name Ysmir is in fact a title which means 'Dragon of the North', and was christened upon the warrior Tiber Septim when the Greybeards formally recognized him as Dragonborn. The same title is also given unto the player character in Skyrim, implying that the name of Ysmir has been given to all Dragonborn.