My guess is Mithril. Mithril full plate is quite powerful, since you have the protection of heavy armor with the mobility of lighter armor. Though, originating from an MMORPG, it might be that color just-because.
Full set means- possibly another (ex)-adventurer who was able to farm/collect/transmog it all themselves, or someone highly-ranked enough to be able to claim any item that drops for others that match their set. If it is another adventurer- my guess is some player who Ellisia brought over that had been acting normal in the game when others were around, but upon waking up here, either thought they got Isekai’d (plenty of anime out there that go that route- a player waking up in another world (sometimes even the real-ified game itself) as their character)(“hey, it’s not real, so I might as well give in to my desires consequence-free!”), or got drunk on power since there weren’t other adventurers around to stop/balance/counter them, and decided to build a strong nation/conquer the world under their rule. From recent re-read info-collation, it sounds like Ellisia is beginning to bring back some adventurers she judged worthy and/or helpful to the re-realized (galaxy? Universe?), but as acknowledged earlier with how bad disappearing the adventurers in the first place turned out to be, her judgement isn’t always perfect, and this particular one turned out to not do/be what she’d hoped.