Mira is an experienced and skilled tactician who commands fierce loyalty from her men. All she wants is to save the soldiers she's sworn to lead, but she sees little hope of withstanding the Orks for much longer, though she refuses to give up.
The following text contains plot spoilers.
The 203rd Cadian Regiment was on route to another war zone when their troop carrier was suddenly diverted to Forge World Graia, as they were the only Imperial Guard force close to the planet. The Regiment is the only force that manages to land before the Orks capture the Orbital Defense Gun, which cuts the Guardsmen off from reinforcements and orbital support.
Lieutenant Mira landed with the rest of the Regiment, which was slowly worn down by Ork attacks. By the time the Ultramarines squad reaches them, Mira is the highest ranking officer left. During their meeting, Captain Titus offers to neutralize the Ork-held gun fortress, which Mira accepts, as the 203rd does not have the numbers to assault it.
She leads the squad through the trenches made by the regiment towards the gun fortress, fighting Orks along the way. She shows them where the Orks have breached the gun fortress and returns to her regiment.