Duo isn't a regular, exactly, but he is something of a frequent irregular. Sometimes he's in twice a day, and sometimes Wufei doesn't see him for a week or more. He usually gets the house coffee with a little cream, no sugar, but he's gone through just about every item on Wufei's menu. If he's coming off a run in the Sister Helen, he either orders a double espresso or camomile tea; Wufei can't quite predict which day will be which.
Duo doesn't like the brighter blends. He'll drink them—but he doesn't enjoy them the same way he does when the house blend is a rich classic cup or one of Wufei's heavier, smoother cups. It's not profitable to stick only to beans that one customer likes—Wufei, to a certain extent, is constrained by what is available and affordable on the market.
So Wufei finds a compromise: he changes the house blend as often as he always has, and makes those changes based on what he has in stock and what appealed to the most customers in the past. But he also keeps a small, special blend for the weeks that Duo wouldn't like the house blend, fine-tuned to Duo's taste. Then, if Duo comes in one of those days, Wufei is 'making a fresh pot' just as he arrives.
It just so happens that that pot makes exactly one cup and is only ever served to one particular customer.
Wufei doesn't lie to himself about his motivation. He accepts it, and moves on. Duo never realizes that he isn't getting the same coffee that everyone else is.
Meilan piloted Shenlong and died trying to assassinate Trieze. Wufei was a scholar, and then he was a freedom fighter. And then there was peace. Now he owns a coffee shop on L2, and is somehow friends with all of the Gundam pilots - even the one he didn't know during the War.
My notes: Fluff with a side of angst and a dash of plot. Wufei is less angry, but no less serious. It's nice to see his relationships with the other pilots grow and deepen, even as the flashbacks explain the differences in how the war unfolded and the friendships started. CW for the suicide of a planet, but if you watched Gundam Wing, you already knew about that.