The First Sword is often the deadliest of the bravos and water dancers of Braavos. The First Sword commands the Sealord's guardsmen and protects him in public. First Swords traditionally serve for life.
A Game of Thrones
Syrio Forel, a former first sword, tells Arya Stark that he had been chosen by the Sealord of Braavos because he was the most observant of the bravos, and most able to protect the Sealord from threats seen and unseen.
Known First Swords
The first sword of Braavos does not run.