Keri is another character that you can romance with. She is a journalist that you will meet several times on Nexus, during which she will interview you - and during the said interview you will be able to flirt with her. She is a bisexual character- you can romance her both as Sara and Scott Ryder
Note - romance with Avela is not a close relationship, it's just a "fling". It means that you can flirt with and romance her without limiting yourself from "real" romances with other characters. You don't have to worry about missing the "commitment point" with her, as there's no such thing this time.
The only moments when you can flirt with the character take place during the Path of a Hero mission available on the Nexus station. A full description of the quest can be found under the Nexus, Additional tasks section of this guide. You can flirt with her each time she records you and asks you questions. Further parts of the interview will unlock once you've completed more Priority ops. Check your mail regularly - she will contact you when another part of the interview (and a chance to flirt her) is available.