Jackson was a soldier from District 13. She was part of Squad 451 and was second in command to Boggs.
After Boggs died during their mission in the Capitol, she demanded that Katniss give her the Holo. Katniss lied to her and said that President Coin authorized her to take charge and to lead a mission to assassinate President Snow. Cressida supported Katniss' lie, which helped convince Jackson to allow Katniss to take charge.
Biography
Mockingjay
Jackson was assigned to be a soldier in Squad 451 as a sharp shooter with Katniss, Finnick Odair, Gale Hawthorne, and other soldiers. Jackson kept the team under control and often gave them orders. Jackson helped Katniss receive the task to guard Peeta during the night. She helped Peeta with his memories by playing "Real or not real" with him so he knew what was real and a lie. Jackson argued over the ownership of Boggs' Holo which he gave to Katniss. Cressida backs up Katniss and Katniss tells her that she is on a mission from Coin to kill Snow quickly so the war can end. She witnessed the broadcast that lead Panem to think that the squad members were all dead.
Death
Jackson and Leeg 1 sacrificed their lives at the meat grinder pod in order to hold off the lizard mutts and allow Katniss and the other members of the team a chance to escape. It is unknown how she died. She was either killed by the Meat Grinder or was decapitated by the lizard mutts. Later, Katniss regrets having lied to Jackson and inadvertently getting her killed but Cressida reassures her that Jackson knew they were lying but wanted to push forward with the mission regardless because she trusted Boggs who clearly wanted Katniss to go on.
Physical description
Jackson was a middle-aged woman who looked kind of sluggish. She was farsighted which gave her the advantage of being able to shoot things that the other members of her team couldn't see without a scope.