From Halopedia, the Halo wiki
In Halo 3, he supplies the voice of Gunnery Sergeant Reynolds, named in homage to Fillion's most famous character. Not only does Nathan Fillion provide the voice for Gunnery Sergeant Edward Buck in Halo 3: ODST, but the character's physical appearance was modeled after the actor as well. Fillion has also confirmed his work as a voice actor in Halo: Reach reprising his role as Buck for a cameo in the level New Alexandria, and also providing the character's Firefight voice. Fillion once again reprised his role as Buck in Halo 5: Guardians.