I'd really need to see that in game to get the full context, but I suspect your original supposition, utunnels, is correct. Let's go with the process of elimination
Knights (or people who could possibly be described as knights) that we know of:
Glen
Glenn
Dario
Viper
Crono
Cyrus
Karsh
Marcy
Turnip
Pierre
Radius
Zoah
The Unused Journey To the West Character
Garai
Solt
Peppor
We can eliminate a few of them right off the bat. Marcy, even though she's a deva, is a she and the barkeep IDs the mysterious figure as a he. Turnip's cape is blue, and it seems like the barkeep would mention if it was a giant turnip. Cyrus is dead and has been for 420+ years. Obviously can't be Viper. Solt and Peppor lack capes and can't be considered powerful looking.
This leaves us with a better selection but we still have very little to go with. The Green Cape might seem important, but we can imagine that a character might not be wearing it when we see them in game (or it could be left over from an earlier design that was changed). Could it be Zoah? He looks powerful, but he lacks a cape. The cape might be explained away as something he wore at the time but discarded later. Though I'd say the lack of pants should have burned the memory into the barkeeps head, so Zoah's fairly unlikely.
Karsh is in a similar situation; lacks a cape (and a sword; knights are often depicted as using either lances or swords) but looks powerful (or at least mean). But considering he's famous, I'd also say he is unlikely.
Garai is very knightish, but also lacks a cape and seems far too famous to go unnoticed. Radius was a former deva and even if he looked powerful at the time and wore a green cape, he too is probably too famous to go unIDed.
Pierre is suitably knightish but doesn't look powerful and lacks a cape. He also seems to be new-come to El Nido at the start of the game.
Crono counts as a knight, would look powerful, but lacks a cape and has no particular reason to make a guest appearance in this fashion.
Glen is, of course, the quinessential knight, but he's out of time: he'd be around 400 years too early to deliver the flag to the barkeep.
There is the unusued Journey to the West Character, but we know nothing about him and my impression was that those characters never made it past the design stage.
Glenn lakcs a cape and doesn't appear too powerful, but he lacks the fame that would trouble other characters.
Which only leaves us with Dario. He's a knight, he's powerful looking, he has a green cape. The only objection, it would seem, is that he "seemed to be absent at that time" and that the barkeep should recognize him.
Utunnels, I'm not sure what you meant about Dario being absent at the time. Does the Barkeep say when he received the flag? If it was at an early date, Dario could have still been around and wouldn't have been famous at the time, so the Barkeep wouldn't have recognized him. Also, what world was this in? To my memory, Dario was attacked by Karsh before the dimensions split, meaning that if he survived in one dimension he should have survived in the other. He might have been a poweful but disfigured individual then, explaining why the barkeep wouldn't ID him.
