Who she is: F.R.I.D.A.Y.
Her power: She’s actually a computer program, so a modified version of J.A.R.V.I.S., really.
Her story: F.R.I.D.A.Y. first appears in Avengers: Age Of Ultron, where she’s chosen from a pile of backup programs to serve as the replacement for J.A.R.V.I.S., Tony Stark’s previous natural-language user interface system. She takes over the computer system duties previously performed by that program—now uploaded to become part of the Vision—and continues to serve Iron Man during the events of Captain America: Civil War. She has quite a lovely Irish accent, incidentally.
Played by: Kerry Condon
Currently, F.R.I.D.A.Y. is: Busy running large parts of Stark Industries.
Where will we see her next? The best bet is Spider-Man: Homecoming, in which Tony Stark is rumored to have a significant role.