Little is known about Helen's time in the CIA, except that for a time she worked closely with a case involving Teo Braga. The case brought her close to Auggie Anderson. Helen faked her death as part of that mission in order to go under deep cover.
Helen tracked Arthur's son, Teo Braga. After she went under deep cover, she continued to meet with Arthur in secret to give him updates on his son. It was during this time that she used the alias Teresa Hamilton. Although this is unproven, some suspect she is the woman with whom Arthur had an affair, and thus, Teo's mother.
Auggie and Helen met sometime prior to the mission that caused the loss of his sight in Iraq. In Space (I Believe In), the details of their relationship were revealed by Auggie to Annie. Helen and Auggie were on a mission to track down Teo and had to marry for the sake of cover, although they apparently later became legally married. She faked her death by appearing to be shot while the two were undercover, so that she could go dark and work in weapons proliferation for Arthur, presumably to get closer to Teo. As part of this, she stayed away from Auggie, who believed she was dead, until the events of Crackity Jones. Helen returned to a troubled Auggie, who was frustrated that she had not contacted him in years, and told him that the marriage was a lie and she never loved him. Helen is very clearly still in love with him, but is willing to let him go, since she knows he loves Annie, telling him they are good together.
However, in No. 13 Baby, Helen invites Auggie via text to come see the apartment where she has been living as Teresa Hamilton. Auggie accepts the invitation. When he arrives, she shows him around and offers him a glass of wine, which he accepts. She apologizes for not being able to reach out to Annie, something she had promised him. She admits that she lied about no longer having feelings for him, and apologizes for causing him pain. He says he is glad she is there, and when she asks him to clarify, he says he is glad she is alive. They kiss, and fall back to the couch, where they make love. When they wake up, she makes him coffee, saying she doesn't want to talk to him (presumably about their sexual encounter) while they're both still in bed. It is clear that she wanted to be back with Auggie, but knows he loves Annie. She states to him that she knew he was in love with someone else but wanted to try to get their relationship back. They are interrupted when Auggie gets a call from Barber.
Annie and Helen have a friendly relationship, although at first Annie worries that Helan's past with her boyfriend will resurface. Annie feels connected to Helen once she goes dark as Helen had before her. Helen tries to help Annie as much as she can, even warning Annie that contacting Auggie could put her in jepoardy.
In No. 13 Baby, Helen meets Annie at her own grave, where a suspicious Annie nearly shoots her. Helen reveals that she knows all the details including the alias Annie took on when she went dark, because she saw Annie in Geneva while tracking Henry, but has not told anyone. She explains that Joan and Arthur don't know Annie is alive and that she has promised Auggie she wouldn't tell anyone. Annie quips that she and Helen are "two ghosts in a graveyard", and Helen asks if she's cracked the cypher she found. When Annie says she gave it to Calder, Helen expresses her faith that Auggie can crack it. Annie asks if Helen is there to tell her to stay dead, Helen says no, but explains that if Annie wants back into the agency, she needs to speak up soon, and reveals that Auggie still loves Annie, telling her not to wait long.
Helen follows Annie and/or Henry Wilcox to the bus station, where Annie is planning to take a bus to New York. Henry has been tracking Annie, believing she is Jessica Matthews, but when Helen steps out from the shadows he turns and then assumes that Helen is in fact Jessica Matthews. Helen threatens Henry with evidence of his misdeeds before (falsely) comfirming that she is Jessica Matthews. Helen begins to tell him it doesn't matter, but before she can finish, Henry shoots her three times in the chest with a silenced handgun. Annie seems surprised when she sees Helen's body through the bus window, making it fairly clear that Annie did not plan for Helen to lie to Henry this way, and that Helen either planned it on her own or decided in the moment to claim the alias to protect Annie; earlier in the episode Helen is clearly distraught over the fact that Auggie has chosen Annie over her, and she also indicates wanting to leave deep cover - it is possible that she saw this as a way out and a way to make up for her past misdeeds.