Who is the leader of the Grey Company that aids Aragorn?

The leader of the Grey Company that aids Aragorn is Halbarad. He is a Ranger of the North and a kinsman of Aragorn, descended from the Dúnedain who guarded the northern lands after the fall of Arnor.

Halbarad joins the Grey Company after Elrond sends word to the scattered Rangers that Aragorn needs their help. The group meets Aragorn in Rohan and travels with him, Legolas, Gimli, Elladan, and Elrohir. Halbarad brings Aragorn's horse, Roheryn, and a standard made by Arwen.

The Grey Company follows Aragorn through the Paths of the Dead. At the Stone of Erech, Halbarad unfurls the banner, an important symbolic moment because Aragorn is claiming his heritage as Isildur's heir. The company then fights at Pelargir and sails north with Aragorn's forces.

Halbarad dies during the Battle of the Pelennor Fields. A common mix-up is identifying Aragorn as the company's leader: Aragorn commands the wider expedition, but Halbarad is the named captain and leader of the Grey Company.