personality
At his core, Duo is easygoing, friendly and confident. He is generally optimistic but with a healthy amount of pragmatism and a little cynicism to round it off. He's talkative and gregarious, which has been known to grate the nerves of those around him from time to time, but that rarely deters him. Due to his wise cracking and joking nature, others tend to underestimate him. Over the years, he's become jaded and prone to darker humor which is something he doesn't seem to mind at all. His friends who know him best know that any stupid remark is a fool's act he puts on for others rather than his own naivete or stupidity. He's very smart, but doesn't believe it himself. He has a low self esteem about his own abilities and intelligence despite his bravado. Over the last few years he has improved on that quality and has started to take himself and his work more seriously. He's also mellowed out as he grew older, but only by so much that he's calmer and more focused on his tasks and things he needs to accomplish.
When he sees people in pain, he wants to do something to fix it but he may not be able to do it directly. He'll work his tail off however he can to fix the problem if that's what it takes. Alleviating the suffering of his fellow colonists was one of the many reasons he agreed to get involved with Operation Meteor until he found out what it's real purpose was. Likewise, it was that which caused him to reject Operation:M and plot to both his mobile suit and himself to prevent a massacre. He is willing to die for his cause if he has to, though that willingness does not mean he has a deathwish.
Duo is a forward thinking person, and dwelling on his past is just asking for trouble so he tries to keep himself in high spirits. He never likes to feel like the earth is shattering around him and the sense of hopelessness frustrates him, so he does what he can to ignore those feeling and put on an act for those around him. Rather than slipping into despair, he buries himself in work or finds an outlet, be it people or physical activity. Being alone only seems to make things worse, so he actively surrounds himself with companions to distract himself from whatever is getting to him. There are times that his usual defenses fail him, so he puts on that smile of is to fool others into thinking he's alright, and sometimes it's even enough to fool himself.
War takes a toll on the hearts and minds of the soldiers and Duo knows that. He's seen so many people change over the years, as well as himself. In the battlefield he's focused and dedicated, yet always the energetic snarker inside of battle as out. The gundam pilots are known for their ruthless efficiency in combat and he is no exception as he defeats his enemies quickly and with skill. He's a verbal fighter, taunting or mocking his enemies while egging himself on. His temper is a slow burn, letting him maintain face for as long as possible, if constantly provoked it builds to an inferno. He's prone to being a bit moody so he tends to avoid situations that will bring him there. He also tends to avoid direct physical confrontation for the same reason. It's never a good idea to pick a fight with him, as there are some very good reasons why he's come to be known as the God of Death.
He's never been much for lying, nor has he ever been very comfortable in it. This stems from all the lies he's endured over his childhood from adults, and he abhors it. He'd prefer the truth, rather than being caught in a lie, or resort to his usual tactic is of evasion instead. It's a lot easier in a war where all you have to do is keep your mouth shut, people usually don't know what he knows unless they ask him directly. He'll only lie indirectly and only if he has to in order to maintain undercover situations which are situations that rarely fall on his lap, fortunately.
Where Heero is the perfect soldier, Duo is the perfect survivor. He will act in ways that will ensure the best possible scenario for his own survival and the survival of those around him. If that's not possible, he may be willing to sacrifice himself for the good of those he's protecting, though he's in no hurry to die. He's very pragmatic when it comes to living and tries not to collect too many things that will just weigh him down. Things he needs will always come before things he wants, and things he wants he'll give away once he moves on. He does carry a certain amount of guilt for his being able to survive where people he knew and loved have died and that weighs heavy on his conscious. Emotionally, he holds people at an arm's length to avoid the agony of loss that will almost certainly follow him if he gets too close. He has trouble with commitment and will probably avoid romantic relationships at first unless he's won over. That won't stop him from playfully flirting, however.