Rachel transferred to Slough after the Swindon branch was shut down. Both Tim and Gareth were keen on her. She is a lot perkier than the other co-workers and enjoys playing pranks on both David Brent and Gareth.
Tim started going out with Rachel as a way of distracting himself from Dawn. Tim and Rachel have similar mischeivous personalities, but Tim clearly prefers pranking with Dawn. Tim later feels that the relationship is moving too fast when she tells him that they're meeting her parents without telling him first. He breaks up with her in the series finale.
Gareth tried getting with Rachel not knowing she was already dating Tim. Gareth couldn't even decide if he should keep persuing her knowing full well she had already gotten with his arch-nemesis. Rachel was fully aware of Gareth's crush and used it to pull pranks on him. After Tim broke up with Rachel, Gareth tried to get with her only to be chewed out.
Rachel wasn't aware of Tim's crush on Dawn and openly flirted with him in front of Dawn, who was coming to terms with her own feelings for Tim. Dawn showed hints of jealousy when no-one was looking and went as far as kissing Tim on the lips at one point. When Tim broke up with Rachel and confessed his love to Dawn, she was clearly torn about her decision to reject him.