Rachel McCord is a Federal Government Agent and former partner of Kate Beckett, when she first becomes a federal government agent.
McCord seems to have considerable respect for Beckett, as proven when she openly lied to their boss, Carl Villante, and even trusts her judgement. Not only that, but she's also a good moral judge of character especially when it comes to intervening and where the federal government cannot do so, being smart enough to not let it become a matter of her own ego.
She has a strict limit to her tolerance for information being concealed, as proven when she became quite upset at Richard Castle for finding out about their case, which she originally thought was Beckett's fault.
This hostility from their first meeting would cause her to have a great disrespect for Castle.
In "Valkyrie" and "Dreamworld", McCord worked with Beckett and Castle against the clock to save Castle from a deadly toxin. When Beckett and Castle followed a hunch on where the killer, Brad Parker was, McCord chose to trust Beckett and followed.
McCord arrived in time to save Beckett from an ambush and thus save Castle's life. When Beckett pointed out how McCord backed her up without even knowing what Beckett was up to, McCord responded with "that's what partners do."
To sum up, she has been shown to be a quite capable Agent and good friend to Beckett. However, when Beckett violated orders to allow an innocent woman to be used as a potential mole to infiltrate a major crime syndicate, instead publicizing her role in recent events so that she would be useless as a secret asset, McCord informed Beckett that, despite her capabilities, she was now fired from her role in the Department of Justice.
She was mentioned in the Season 8 premiere, where it was stated she had died just the day before. She died in a car crash along with three of her team members. Her last team member was stabbed to death at the beginning of the season premiere, providing the first hints of the existence of 'LockSat', an unidentified conspiracy that has some connection to the funds that were being tapped by Senator William Bracken as part of his presidential campaign, leading Beckett to the realisation that the attacks were triggered as a response to an old search algorithm she set up in the government databases when she was working as an agent.