Tristan is the oldest of three, the other two being twins, Ryan and Sandalio. Nobody remembers which one came first, but it doesn't honestly matter anyway. They are technically the children of the former rulers of the Andascan city-state of Calabria, indirectly making them princes of a sort, but they never lived that way. Calabria was one of the Andascan city-states that was at war with the Republic first, and it was very obvious that things weren't ever going to work out, and it was just a matter of time until the Republic crushed them.
Their parents did the best they could do prepare them for that day, in hopes they'd survive. Well, they did survive. Calabria was gassed with dysphoria canisters, and Tristan took the twins and ran in the first direction that came to mind. They ended up running so far, they ran straight into the Dome, but nobody seems to have noticed or cared.
Both Ryan and Sandalio ended up poisoned just enough by dysphoria that they're slowly dying. Tristan doesn't seem to have gotten it, but who knows. Maybe in a few years it'll rear its ugly head. In the meantime, Ryan became determined to join the United Republic. Sandalio was angry about it for a while, but eventually saw the merit in Ryan's intentions, and also joined the Republic. Tristan was amused by this turn of events, but Tristan's remained an Outlander, as he's doing the best he can to find a cure to dysphoria.
Unfortunately, he's starting to get the idea he may have no choice but to brush elbows with the Republic for that, because so few in the Outlands understand how dysphoria is even made, let alone how to counter it. It's tough work, and Tristan still isn't fond of the Republic, but if it saves his brothers, maybe it's worth it.