Jannah is a human former First Order stormtrooper, designated TZ-1719, who leads a group of deserters living on the ocean moon Kef Bir in The Rise of Skywalker (2019). Naomi Ackie plays her.
The company that refused
Her account is the film's most interesting piece of worldbuilding. Ordered to fire on civilians during a ground assault, her entire company — Company 77 — put down their weapons at once. She cannot fully explain why; she says it felt like a decision that was made for them, which the film gently implies was the Force. The significance is that Finn's desertion in The Force Awakens was not unique, and the First Order's conscript army has a structural fault.
Taken as a child
Like Finn she was abducted as an infant and raised as a soldier, and she does not know where she came from. The film pairs her with Lando Calrissian, who offers to help her find out — a quiet scene that gives the sequel trilogy's stormtrooper-conscription premise the follow-through it had otherwise skipped.
What the film does with her
Not enough, honestly. She fights at the Battle of Exegol on orbak-back and is a fixture of the finale, but she is introduced in the last third of a trilogy's last film and the material does not have room for her. The name TZ-1719 comes from The Rise of Skywalker: The Visual Dictionary rather than the film itself.