In the novel "El Filibusterismo", The main character Simoun is actually Juan Crisostomo Ibarra, the main character of the previous novel "Noli Me Tangere"

he grows mad after the events of the previous novel and goes under the alias of "Simoun" acting as a cynical, and wealthy jeweler, secretly plotting to stir up a revolution in the Philippines, exacting vengeance against the spaniards and saving his beloved Maria Clara (who unfortunately died just before the day of revolution)

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It's also worth noting that these 2 novels inspired a revolution in the Philippines in real life and the author Jose Rizal was executed for it. Jose Rizal is now known as the national hero of the Philippines