Elyo Grivas

Elyo Grivas was a First Sword of Braavos, some time before Syrio Forel.

Behind the scenes
There is no character named "Elyo Grivas" in the books.

The character is blatantly named after Elio Garcia, who runs the Westeros.org fansite, and who co-authored the World of Ice and Fire sourcebook along with his friend Linda.