Note: History is inferred based off what can be translated, and may not be entirely accurate.
Not much about Rin's past is known, except for her seemingly legendary track record as an assassin and fully-trained Pilot to the point that she is often listed as a potential suspect in any high-profile assassination on the Frontier as a matter of course. However, all attempts to find or capture her have failed due to lack of evidence and her natural elusiveness - though she is known to employ a customised Stryder-class Titan christened the "Black Mamba" that's just as elusive as she is.
Eventually, Rin attempted to show off her skills by leading an assassination attempt on multiple core IMC personnel, an attempt that ultimately exposed parts of her mysterious past. According to the information revealed, the IMC once spearheaded a particular covert "human enhancement" project, a project that Rin was a test subject in. The experiments failed and the project was scuttled, but Rin somehow survived. The barcode and serial number on the side of her head may be a remnant of her time as a test subject. Now under the alias "Hollow," she ultimately seeks to exact revenge on all those involved in the project.
"Hollow" prefers to work alone, though she's not above accepting help from others if it serves her needs. Part of the mythos that surrounds her (and her name) is not just her frightening prowess in the art of assassination, but also just how easily she disappears from a scene like a ghost. These and other skills, combined with her Pilot training and maneuverability with her Stryder "Black Mamba," are hallmarks of her assassination style and why she is a terror on the battlefield.