Cossac Gundi/Aiden Aslin, originally from Britain, was a volunteer fighting ISIS then later signed up for the UA armed forces and served in a Marine Bn some time before the big Russian invasion kicked off - IIRC first in an infantry role then mortars.
He was part of the defence of the Illich steelworks in Mariupol (not Azovstal) and opted to surrender to the Russians and was eventually released after captivity. He has written a book about those events and I guess especially the captivity - if it hasn't come out yet it's about to.
He has participated in some podcast interviews, not much detail on UA Marine Bn organisation but might be interesting nonetheless:
https://player.fm/series/battlegroun...ge-of-mariupol
https://player.fm/series/popular-fro...oples-republic