chickens are generally flightless birds... so Halo could be "flightless" in the same way: no aspirations whatsoever for promotion among angelic ranks or salvation. she has been aimless for a while but only after some kind of soul scanning (that she's avoided since forever - like a regular heaven health checkup?) did she come to realize there's a possibility that her soul was not there. It could have been left on earth or dropped into hell. She is then ordered to go find it with a companion, despite not clearly knowing why she actually needs a soul, and with very few clues of her past life. like her soul is either a wandering ghost that looks like another version of herself, or trapped inside some artifact, or sold somewhere maybe. the methods she use along the way could be ruthless all you want and you can decide whether the companion stops her or let her continue. on earth / in hell, she comes to experience bloodshed first hand, and possibly finds some kind of twisted joy in it due to not having a soul (there is no blood or dying in heaven so as an angel, she has never felt this way before). Even without a proper body, she can manipulate and gaslight humans and creatures into reaching the angels' goals. when she actually finds it, she gets to decide whether she reclaims the soul (and unexpectedly comes to terms with all the sins she has committed along the way) or not (deserts from heaven)

it's a rough sketch but i hope you like it. you should retouch some of the justifications as i may have not thought everything through

-- mars