Sage is a term used to identify the reincarnation of Aita, a member of the First Civilization, inside of a human being. Aita was the husband of Juno. There's a joke about the binding shackles of marriage here, but I'll leave the jokes to you. Databases are serious business.
From what I understand, Juno managed to manipulate human DNA so that his consciousness could re-emerge periodically, almost at random. What this means is, a person will be born with their own personality, but they will eventually come to possess the memories of Aita. Most Sages possess a distinctive identifying trait: heterochromia.
Often the Sages bear a strong physical resemblance to Aita, and to each other, and to date only male Sages have been identified, but we believe that there is a small chance that female Sages could also manifest.
(My cousin Mark has two different-colored eyes! We should take some of his blood the next time we're in Vancouver! You know, just to be safe. -RC)
Sages are valuable to Abstergo because, according to them, a normal human genome contains roughly 0.0005% of First Civilization DNA, but a Sage can have up to from 5% to 6% in their genomes. That doesn't sound like much, but the more samples Abstergo obtains, the closer they get to sequencing a First Civilization genome, and unlocking their powerful artifacts.
Back before we could more easily identify them, there were Sages that managed to sneak their way into the history books, like the dreaded pirate Bartholomew Roberts, and even a couple of Templar Grand Masters: Jacques do Molay and François-Thomas Germain. More recently, an Abstergo Entertainment employee named John Standish tried to use their technology (and a helpless fleshy "volunteer") to bring back his beloved wife.
(Do you think there's ever been an Assassin Sage? -RC)
It looks like you found another Sage operating in World War I, this German spy master. But honestly, by the looks of it, he wasn't one of the better ones. Still, when things quieten down, we ought to send someone to see if there are any of his remains left in the city.