Mon Mothma is a fictional female Human in the Star Wars series. The character was portrayed on screen by Caroline Blakiston. Her allegiance / loyalty is to The Rebel Alliance. Mon Mothma is one of the principle leaders of the Rebellion. Mon Mothma is seen briefly in the prequels but is most remembered for detailing the attack on the unfinished Death star and on the shield generator on Moon of Endor. A younger Mon Mothma is also seen in a deleted scene from Star Wars III - Revenge of the Sith where she is discussing the state of the Senate with Padme Amidala.
Mon Mothma returns to the film saga in Star Wars - Rogue One where she is again leader of the Rebel Alliance when she tasks Jyn Erso with a mission to bring in extremist rebellion leader Saw Gerrera. After Jyn has returned, there is a Rebel Council meeting where the Rebellion decide not to go after the Death Star plans prompting Jyn Erso, Cassian Andor and a group of other rebel fighter to steal the stolen Imperial cargo ship and go to Scarif.
Mon Mothma is no fighter, she is instead a politician. Before the Galactic Empire, she was a member of the Senate. In the original trilogy, she was played by Caroline Blakiston, who was fifty at the time. In Rogue One, she is played by Genevieve O'Reilly who is 39 so is not far off how old Caroline Blakiston's character was supposed to be in Star Wars VI - Return of the Jedi. Events in Return of the Jedi took place many years after the events of Rogue One.