Appearances in CoD:
In Chapter 5, he is first captured by Carlito Flores and presented to General Salazar as the rebel leader. He is apparently the leader of a rebel group against Salazar, a San Trobidan tyrant who kidnapped Mal. Carlito makes a unique and odd move by hiring Mal to be the one to interrogate Gabriel -- earlier in that chapter, Dr. Luis Villareal launched a plan that Mal would use a pin he gave him to escape the interrogation room and then run out the room.
During the interrogation, as Mal weakens his handcuffs, hell breaks loose as Esmeralda's guard (who is watching over the interrogation in her place to ensure that Mal doesn't tell Carlito the truth), begins to fight him. Mal and Soto reach the gates after they escape, but only one of them can be able to escape before the gunmen come in. Mal, knowing Gabriel can save rebels, lets him go as he is subdued by the forces of Salazar.
In Chapter 7, Soto helps subdue the rebels who are styming Mal and his father. He later agrees to help fix the plane that Jacob Fallon used to get to San Trobida after it's pipeline was caught, but when it's fixed, Mal, wishing to save lives and defeat Salazar, refuses to leave at that point. He also uses a quote Soto once told him ("Better to die a hero than live a coward") to convince his friends to assist him in his battle. It is also commented that Jacob Fallon once worked a smuggling operation with Soto as a corrupt cop.
In the closing scene of the highly climactic Chapter 8, Soto tells Mal that while many rebels have been displaced and injured and that San Trobida will take years to rebuild, he will still remember the late Jacob Fallon's efforts for saving it.