Shalem is good at covering his tracks. I don't usually employ people I don't understand, but his skillset was too useful to pass up. He's obviously using us as a means to some personal end — we'll see who shows their hand first.
Shalem was a licensed assassin with a perfect record and an ego to match. He specialized as a sharpshooter, and would track targets for months, patiently waiting for the perfect shot. Enjoying almost celebrity status, he was able to finance a lavish lifestyle from his contracts. Why he turned away from a life of fame and riches to work for Invisible is an open question.
Shalem's mother was a conscript surgeon during the opening years of the Resource Wars. When she lost the use of her right hand in a mortar attack, she trained young Shalem to wield her scalpel for her. While his true passion lies in ending life, he still remembers how to preserve it.
Some of Shalem 11's backstory is described in his datalog.
At the start of a mission:
More of Shalem 11's dialogue can be found on the agent banters page.