Duo first met Heero when he shot him, thinking that he was trying to hurt Relena, who was also there. He then stayed with Heero throughout most of the war, helping him (or trying to) to fix his mobile suit after Heero resurfaced it. At one point, Deathscythe was destroyed when it was taken and Duo was fleeing from the OZ corporation. This was the only point (it seems) that Duo was ever at the depths of despair which says something about his personality and outlook on life. He also was the pilot uses to try out the Zero System for OZ and it took him over while he was using it. Duo also was the one to find Trowa in the circus after he had lost his memory. Duo tried to help him get it back. In the final battles, Duo fought for Peacemillion with the other pilots and was the one that found and freed the five scientists which were forced to work for OZ at the time.
In Endless Waltz, Duo caught up with Sally and Noin to work against Mariemaia's army. Deathscythe was returned to him and he fought with them in the battle against the enemy. In the end, he went back to live with Hilde and work in a garbage dump that he had previously called home.
I guess it's time for the DANDIN MATRIX RATING SCALE