That leads us to another topic: how Nanashi came to have the name Trowa Barton, but before that came a time after Midii's presence and yet before the real Trowa Barton's. As mentioned earlier, Trowa, at the time Nanashi, had no recollection of his past much less his name.
After Midii's betrayal time went on and according to Blind Target he joined one more Mercenary Corps. before heading to space. He was around nine or ten perhaps. He stayed there for a few years and he met a man known as Ralph Kurt, but they were mainly just acquaintances at most. Eventually that group was eliminated as well. During Blind Target there are hints about things though. Underlying meanings come up about his name and Heavy Arms.
When all of the Mercenaries were eliminated Trowa went to space and ended up in the L3 Colony. There he worked under the Barton Foundation as a mechanic in AC 195. There, as Nanashi, he met the real Trowa Barton who was three years his senior. Not much was known about their relationship but it is claimed on the Internet that Trowa liked to bother Nanashi and thought he was stupid while Nanashi never liked, perhaps even hated, Trowa.
While Heavy Arms was being built, it was found that Operation Meteor, which was what the Gundam Suits were for, was actually an operation planned by the Barton Foundation to drop a colony or two on Earth that would kill countless people. Doktor S and an associate who had been working on Heavy Arms' creation opposed this and Trowa Barton ended up being shot and killed.
Nanashi had witnessed this and was discovered by the doktor. In the end Nanashi took up the dead man's name and became the new pilot of Heavy Arms.
The Silencer: Trowa Barton. Information on "real" Trowa Barton.