Sotillo is a colonel who joins the military uprising against Ribiera, seizing control of the garrison at Esmeralda and then moving up the coast toward Sulaco. A healthy chunk of the book is spent describing preparations for Sotillo's arrival in Sulaco.
Once he gets there he's not super inclined to seek out Montero or cede decision-making/control of his own revolutionary activities to him. In fact, like Gould and Nostromo—and seemingly everyone else who gets involved with the mine—he becomes obsessed with figuring out what happened to the silver that was supposed to be in the Custom House when he arrived (which he had planned to steal, naturally).
This obsession eventually leads to his downfall, with some encouragement from Doctor Monygham. In an effort to keep him distracted (i.e., less interested in joining Montero/pillaging Sulaco), Doctor Monygham encourages him to embark on an extended (and fruitless) search for the silver, leading him to the absolute wrong place to search.
Sotillo ends up dying because of this obsession. Barrios's men arrive in Sulaco and come across Sotillo on his boat trying to drag the harbor for the precious cargo. They waste no time in attacking Sotillo's steamer, and one of Barrios's men runs him through with a sword. Anyone keeping track of the silver's body count yet?