Tío Luis and Tío Marco are the big, bad wolves of this story. Well, according to Esperanza, they look more "like two underfed billy goats" (2.57). But that doesn't make them any less mean. Although they aren't around for much of the story, they're the reason Esperanza and Mama have to leave Mexico in the first place.
These guys are the two most powerful men in Aguascalientes, but that doesn't impress Esperanza or her mom. Tío Luis, the ringleader, is the owner of the bank and a candidate for governor. Tío Marco is the mayor of the town, but he "always followed his older brother's lead, like un burro, a donkey" (2.57). (Are you enjoying all these animal comparisons? We sure are. And they're probably supposed to indicate something…)
The two tíos let their power go to their head, that's for sure. Even Papa, their own stepbrother, says that they're still single "because they loved money and power more than people" (2.57). (Their looks probably don't help, either.) Exhibit A: hoping that Mama's influence will help him win the election for governor, Tío Luis tries to bully her into marrying him. But luckily, Mama is a strong woman, and she refuses to marry that ugly old goat.