Bucho is El Mariachi's half-brother.
Either that or their parents were very different looking, and each brother took after one parent. Or one or both of them were adopted. El Mariachi's American friend is Mr. Pink
(A fan threory this troper read on another site and cannot take credit for.) After making his getaway with the diamonds, Mr. Pink fled to Mexico. Fate sadly caught up with him and despite his best efforts to die in bed, he wound up joining the rest of the Dogs courtesy a Danny Trejo. Unless... The American isn't really dead
Those knives were barely in his chest. They probably didn't puncture any organs. They might not have even punctured the skin; a guy as paranoid as him might wear a bulletproof vest. (In Mexican heat? Why not? He and every other male character wear long sleeves and long pants.) Upon being hit, he played dead on reflex and waited for the trouble to be over, then got the hell out of there and decided never to talk to El Mariachi again. He eventually hooked up with the irritated female customer who told off the bartender. She enjoyed his kickass if obviously BS stories and loved how he scared obnoxious bartenders with them, and fell in love with him. However, their relationship wasn't perfect, as he always wanted to go to places with beer that she thought tasted like piss.