We know what Gage did before joining the group of Assassins, and his past story is similar to Dez’s in that they were both former, highly-skilled military personnel.
But, all we really know about Julien’s life before the guys is his high intellect.
Is he former military? Could be.
Is he a computer nerd turned somewhat-hero? Maybe.
Was he part of what could be described as an underground, pseudo-anarchist group whose mission was to dismantle the infrastructures of sovereign nations? ::shrug::
Guess we’ll have to wait and see.