The chap wearing the armour is Nik Sant (AKA 'Gramps'), one of the Endor Strike Team who assaulted the base.
The armour belongs to the imbecile pictured below, listed as "Unidentified Scout Trooper" in the credits but almost certainly played by Peter Diamond who also portrayed a Stormtrooper, a Death Star Trooper, a Snowtrooper, a Scout Trooper and a Rebel Commando.
It may interest you to know that the film's novelisation states that the hapless stormtrooper was rendered unconscious rather than being killed:
Back at the bunker, the last scout was undone. Subdued by Chewbacca, bound, stripped of his suit, he was being carried into the woods now
by two other members of the strike team.
Leia peeked inside. No sign of life. She motioned the others, and
entered the bunker. Han and Chewie followed close on her heels. Soon
the entire team was huddled inside the otherwise empty steel corridor,
leaving one lookout outside, dressed in the unconscious scout’s
uniform. Han pushed a series of buttons on the inner panel, closing
the door behind them.
It may also interest you to know that in a bonkers retcon, this character is tangentially confirmed to be Rex, a clone trooper from Star Wars Rebels
"…I think the one thing I have really thought about is I really do
think that Rex is that guy on Endor. I really do. Why else is there a
bearded old guy on Endor, Tano? Why? It makes no sense. If you don't
want that to happen, do you know what that means? I'm gonna make that
happen. I'm getting like Palpatine, I'm getting power crazy."
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