Commander Dot Pyxis.

Pyxis was the Garrison Regiment's top Officer of the southern territory, which included Trost District. He was often simply addressed as "Commander" and in matters of defense, he was given full authority.

The Battle of Trost District:

When Wall Rose was breached, and Titans started invading Trost, Pyxis immediately headed over to the scene. Along with Eren Yeager, Pyxis went on top of Wall Rose, introduced him to the soldiers in order to give them hope, and announced a plan to seal the Wall with Eren's Titan power.
Unsurprisingly, Pyxis was not taken seriously and the soldiers turned to leave in despair.
But finally, Pyxis gave an order to pardon all deserters, and stated that by leaving, they will put their families at risk and expose them to the same terror that paralyzed themselves. This argument convinced the soldiers to reassemble their ranks and begin the operation.
After a long fight and with many casualties, Humanity claimed it's first victory against the Titans and the Wall was sealed.

The Uprising Arc:

Commander Pyxis received a letter from Erwin, informing him that he plans to orchestrate a rebellion against the Royal Government.
After speaking with Erwin, Pyxis soon devised a plan to determine whether or not to execute the coup. He decided to test whether or not the current Government is worthy to continue operating, by faking an announcement that Wall Rose has been breached during Erwin's trial.
If the Government Officials elect to allow refugees into Wall Sina for shelter, despite the food shortages that would be caused, then he would turn himself and all of his accomplices over to them for judgement.
If the idea of Civil War and the endangering of their own livelihoods leads them to turn away the refugees however, the military will know that they are unfit to lead and must be replaced, and so it happened.
Pyxis announced everyone's role in rooting out the corruption present within the Royal Government. Erwin was then released and the Officials were taken into custody for valuing their wealth over the lives of the people.

Final Arc:

After hearing that Darius Zackly was assassinated, Pyxis was named as the acting Commander-in-chief of the military.
After Zeke activated his scream from the Forest, Pyxis suffered a reaction alongside with those who drank Marleyan wine and he came to the realization that Zeke had tricked them and made them drink his spinal fluid.
Later in the Battle of Shiganshina against the Marleyans, Commander Pyxis instructed that those who didn't ingest the spinal fluid should be given first priority to the limited number of vertical maneuvering equipment available while those who drunk the tainted wine should follow him to the front lines for a final charge. In his final moments alongside his men, Pyxis was drinking his last bottle of alcohol in shock, as he heard Zeke's scream he then closed his eyes before transforming into a Titan, accepting his fate.