(from Wookiepedia: The Star Wars Wiki)
Serchill Rostok, nicknamed “Rostok the Rock” was a human male soldier that served in the Alliance to Restore the Republic‘s Special Forces during the early rebellion against the Galactic Empire. A renowned sniper, Corporal Rostok was one of the many volunteers for Captain Cassian Andor’s unauthorized mission to retrieve the technical readouts for the Imperial Death Star superweapon from the Imperial security complex on the planet Scarif in 1 BBY.
Armed with his signature A-300 blaster rifle in a sniper configuration, Rostok was a member of a distraction team that drew away Imperial forces from the complex and allowing Andor’s small infiltration team to retrieve the plans. Although the mission was a success, Corporal Rostok was killed during the battle with the rest of Rogue One…
Read more at: https://starwars.fandom.com/wiki/Serchill_Rostok