Jokes aside, I think it’s probably just a Master Chief reference. Master chief has fought in tons of wars for practically all his life.
This strongly leans in favor of Master Chief.
But if it isn’t a Easter Egg.
Then I’m sure the battle that the Guardian is referring to is the Collapse or possibly the Twilight gap
Mark of the Exile reads: “Though the City won a great battle at Twilight Gap, it lost an even greater mind.” - Sister Lupe.
Murvaux Type 0 (Titan legs) reads: “Kei-Ying: Gave his last full measure at Twilight Gap” - The Last Stands of First Pillars.
Could those help indicate who is in there?
It does narrow down the possibilities, thanks
I’ll do some more digging later
I appreciate your use of an item description! 
I’m pretty sure Kei-Ying is just a name drop for the sake of worldbuilding - a member of the First Pillar order with no other mention in the lore.
Mark of the Exile is Trials gear. That said, it’s more than likely talking about Osiris, who was also at Twilight Gap, and would certainly qualify as both an Exile and a great mind. He didn’t die of course, but the City did lose him, and with the information from that Titan Mark, we can consider the possibility that Twilight Gap might have been some sort of trigger that lead to him leaving.