I'll unload all of the info I have on her. I think she's pretty cool, actually. Quite like her character design.
Argento is a Tsviet, but she doesn't appear to have a title, though the first cutscene in which she appears and talks to Shelke is titled "Silver and Transparent".
She was never given an H-J virus implant, apparantly because the Restrictors trusted her. This allows her to engage in subterfuge along with Shelke.
She acts as something of a teacher or instructor, involved in guiding players through some of their quests. She doesn't fight, and is said to embody the phrase, "strength hidden in weakness."
Her weapon appears to be a fancy, decorative version of the standard SOLDIER broadsword, with an engraved hilt and some unique touches on the blade, including a pair of ? patterns.
She forged Weiss' lever-action sword revolvers, which she presented to him when he was released. The most difficult TSV (Tsviet) rank mission in Multiplayer Mode, "Emperor Unchained", has a description along the lines of, "Weiss, wanting to stretch his legs and try out the weapons Argento forged for him, prepares to fight a squad of Tsviets."
Argento's character design sheet describes her as a subordinate SOLDIER of Weiss. Nothing else interesting.
Nobody knows what happened to her. She might still be alive, she might be dead. Since there are still secrets to be revealed about Genesis and his connections to Deep Ground and specifically Tsviet, she might still be of use, so I hope she isn't just completely forgotten about.