She is an "AI" that is linked to the Hyperion network. She helped the original Vault Hunters and did so as well for the second set. She can unlock or access anything connected to Hyperion on Pandora. She appears through the game by communicating with you via your ECHO. She is the one who points you to the body double and helps a lot in trying to topple Handsome Jack
She is found in The Bunker, a connecting area of Thousand Cuts. You can make it to "Control Core Angel" only after getting through the Death Wall, breaking into the Bunker and opening a door that will only open for Handsome Jack. The Impossible has never been so easy.
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Angel is the Teenage Daughter of Handsome Jack, then known as John. She is 1 of the 6 Sirens that inhabit the known universe. She, unlike Maya or Lilith, does not have a Phaseshift that really effects the physical world. Her's, as seen throughout the game, is focused on the digital. This lets her hack and sort digital data that is connected to her network.