Origin
Kali Vries is a SHIELD Agent and an old friend and lover of John Walker (U.S.Agent). The two first met in army boot camp, long before either was involved in espionage. They would compete in training exercises and she delighted in being able to best him more often than not.
U.S.Agent mini-series
Following the “Maximum Security” event, Walker was assigned as leader of the new STARS organization (Superhuman Tactical Activities Response Squad). Vries was assigned as his co-leader, apparently thanks to the influence of Senator Warkovsky. Walker was concerned that she was after his job. He became further displeased following their first mission together. They were to capture Machete, trying at the time to sell mind-control organisms of unknown origin to HYDRA. Walker managed to capture their man but HYDRA security caught up with them. Vries helped Walker escape safely but also killed the captive mercenary with a bullet to the head. She claimed it was an accident while in conflict.
The two kept investigating the case of the mysterious organisms. But Warkovsky was the one behind their distribution and Vries was aware of it. In fact, the Senator assigned her to place one of the creatures on Walker himself. She started trying to seduce Walker while seeking an opening to make her grand move. She thought she succeeded and then reported back to Warkovsky. Kali thought she had gained his trust and wanted to find the source of the creatures. But the Senator did not trust her enough for that. She then contacted Captain America and set out to rescue Walker.
They followed him to the supplier of Warkovsky, the Power Broker. The Broker had become the host of an alien and these organisms were her “children”. The rescue mission soon turned into a fiasco. Walker had only been faking being under the influence of the Broker. But the Broker placed an organism on Vries and used her against Walker. He both defeated her and freed her from the mind-control. The three agents then easily took care of the Broker. Dum Dum Dugan informed Walker that Vries was supposed to earn Warkovsky’s trust and find out who supplied him with the organisms. The death of Machete was truly an accident. With the end of her mission, Vries left STARS behind. She tried to contact Walker and re-establish the broken trust between them. Walker was too hurt to accept and burned up their old pictures.