Genre Savvy: Unloading Tuco's gun the night before the showdown did the trick.
Good Is Not Nice / Not So Different: He's basically as much of a scoundrel as Tuco or Angel Eyes (and the one to blame for the destruction of his and Tuco's partnership), but retains a moral value unlike the other two and most of the time does what's right, as long as it doesn't interfere with going after the gold.
Jerk with a Heart of Gold: The more we get to know about Tuco, the more we understand that he's one of these. Justified since Tuco is the only character whose backstory we learn; Blondie's and Angel Eyes' histories are a complete mystery.
Made of Iron: Among his several mishaps - getting thrown off a horse by a cannon shot, getting hit from quite a distance by a cannon shot, nearly hung for his crimes on several occasions (especially during the finale), has the hell beat out of him by Wallace (which includes having his eyes nearly squeezed out), and leaps out of a moving train and down a steep hill in an act that breaks Wallace's neck.
"See you soon, id-id-idi..." "Idiots." "It's for you."
"Un-k... un-k... there's no name on it!"
Nice Hat: For a good portion of the movie, although he loses it following the episode in the second Union camp.
No-Holds-Barred Beatdown: Receives an extremely painful one from Wallace. He gets his revenge after springing from the POW train and breaking Wallace's neck. Even after Wallace is already dead, Tuco continues to slam his head into a rock, probably still not satisfied that Wallace's death could level up to the savage beating that he took from him earlier. As if that was still not enough, Wallace gets run over by a train during Tuco's attempt to free himself from the chain binding him to Wallace.
Oh Crap: Has three moments - the first when he realizes the true colors of the soldiers whom he has mistaken for Confederates, the second when he notices that Blondie has set up a hangman's noose for him in the cemetery, and the third is his reaction when he thinks Blondie is going to shoot him rather than cut the noose.
Ax-Crazy: He is actually a subversion; he is more ruthless and evil than crazy, but still meets several traits of a true Ax-Crazy: he is The Dreaded, extremely sadistic and incredibly sociopathic. Even he is seen with a maniacal cackle when he kills his boss.