Who was Restalaan and why should you care? I'm glad you asked. Restalaan was the closest thing Velen had to a buddy - a close friend and confidant, a right hand who led the defense of the draenei city of Telmor. Having been with the prophet since the exodus from Argus, Restalaan was technically an ancient eredar, born during the time when Velen, Kil'jaeden and Archimonde were the triumvirate and led all eredar before the coming of Sargeras.
Restalaan was always there, serving as Velen's sounding board and anchor. In the history outlined in Rise of the Horde it was Restalaan who led the patrol that saved Durotan and Orgrim Doomhammer when they were children - an event that should still have happened in the Warlords of Draenor timeline, since both Durotan and Doomhammer are confirmed to be alive. In the original timeline, Restalaan would come to have reason to regret this decision, since in the process of saving the two young orcs and leading them to Telmor to meet with the prophet he exposed Durotan to the use of the Ata'mal fragment Leafshadow. This enabled Durotan to later expose Telmor to the Horde, which destroyed the city - Durotan killed Restalaan during the battle. So died Restalaan, captain of the guard of Telmore, Velen's best friend, survivor of the exodus from Argus - killed by the boy who's life he saved.
But that fate is not set on the Draenor of Warlords.