After her mother, Bunny, got involved in a high-profile case and was arrested, she was poached for a job offer as the only way to release her mother and so she took it.
Hank fostered Erin at the age of 13 by taking her off the streets and away from Bunnie, who was an addict and encouraged Erin to live the same life. Erin mentioned something about Camille not being happy with fostering Erin when they brought her home but eventually ended up loving her a lot, which she misses.
Hank is very protective over Erin, something that annoys her at times. He told her not to date Jay when they started getting close and though she didn't want to listen to him on this, she didn't until she left Intelligence and joined another taskforce temporarily. They kept going in secret when they came back and he knew but didn't stop them then. He also told Jay later on that he trusts him with her.
Hank is very proud of her and more than considers her his own daughter. He likes that she followed in his footsteps, especially because Justin didn't change his lifestyle until it was too late.
In Pilot, Jay was the newest to the team after being brought in by Antonio. He was extremely wary of both Hank and Erin and didn't trust them. He then found a file of her that listed her offences and mughshots. He said she was a CI and asked about the files and she told him Hank took her in at a young age and that she's had a tough life.
She and Jay started getting closer but Hank told them they couldn't date while they worked together, so they didn't. In Season 2, she left Intelligence after a job offer to join another task force. They started dating as they weren't working with each other but she quickly came back because she hated the new job. They kept on dating without telling Hank but he knew and didn't stop them this time.
Jay moved out of Erin's apartment in Little Bit of Light after he found out that he was still married to a fellow ranger, Abby McSweeney, after believing for the past few years that they'd been divorced. Erin didn't like this but it was Jay who left to live with Will to figure things out but, they were still dating.
After that their relationship was unclear. Jay mentioned to Erin that he knew a couple who lived in separate houses and were very happy, implying he still wanted to be together, but he backed off when she asked him straight up why he was telling her that.
In Army of One, Voight decided to break up Jay and Erin's partnership. He partnered Jay with Alvin and Erin with the new Hailey Upton. Jay wasn't happy with it, but Alvin later told him that Voight should've made this decision a long time ago. He said that working with Erin clearly didn't work, but Jay said he disagreed with that. Alvin told him that it was best for him and his career.
Barbara "Bunny" Fletcher is Erin's birth mother. They all live in Chicago, and when Erin is in particularly tough situations, she usually ends up back with Bunny, who encourages her to drink, take drugs and stay away from Hank, who she believes is the reason that Erin left her. All of their reunions usually end up in fights because of the numerous lies that Bunny tells Erin.
Bunny was first seen in Call It Macaroni, after numerous attempts at reaching out to Erin, calling her at work constantly until she went to see her. Erin wasn't at all happy to see her. Their conversation started with Bunny saying she was getting married, to which Erin replied sarcastically, "Good for you." She said she was trying to clean up her life and Erin seemed really disinterested in all of it. She told her to stop calling her work and email her instead and left.
Justin was Erin's foster-brother.
Nadia was a young woman and addict that Intelligence found during a case in season 1. Erin was harsh towards her in the beginning but Nadia was clearly struggling, so Erin helped her alongside the case and then dropped her off to a rehab centre, saying she'd leave and it was Nadia's decision to go in or not. She didn't see what happened but drove off crying.
Nadia didn't take the help and they ran into each other again. Nadia asked for Erin's help again and she dropped her off to a rehab centre. She then let Nadia live with her at her own apartment and even got her a job as their 'receptionist' at Intelligence. Everyone took a liking to Nadia, even Trudy.
Nadia was murdered in season 2 by Gregory Yates a day after Erin's birthday. This hit Erin very hard and to the extent that Erin flat out quit her job and went to live with Bunny. She stayed this way for presumably a few weeks or months until she came back in Life is Fluid to save Jay.
- (To Hank Voight): "I was born into bad news, and can't seem to get away from it."
- Her mother was a drug-user and neglected her and her brother until Hank took in and fostered Erin at the age of 13.