Overwatch's 31st hero is SIgma, the 62 year old Astrophysicist, but he is a Tank in-game: "Sigma is an eccentric astrophysicist and volatile tank who gained the power to control gravity in an orbital experiment gone wrong. Manipulated by Talon and deployed as a living weapon, Sigma’s presence on the battlefield cannot be ignored." Sigma serves Talon, the main antagonistic faction in the game. You can read his full story here.

You can see his abilities below:

HYPERSPHERES

Sigma launches two gravitic charges, which bounce off walls and implode after a short duration, damaging enemies within a sizable radius

EXPERIMENTAL BARRIER

Sigma propels a floating barrier to a location of his choosing. He can dismiss the barrier at any time.

KINETIC GRASP

Sigma freezes incoming projectiles in midair, converting them into shields.

ACCRETION

Sigma gathers a mass of debris and flings it at an enemy to knock them down.

GRAVITIC FLUX

Unleashing his full powers, Sigma takes flight, lifts enemies in a targeted area, and launches them into the sky before slamming them back down.

 You can also watch his origin story in the video below:

[NEW HERO – COMING SOON] Sigma Origin Story | Overwatch

Jeff Kaplan, Blizz's VP,  also provided "some insight into his back story, abilities, and overall strategy" in a dev update:

Developer Update | Introducing Sigma | Overwatch

Sigma is currently available on the PTR.