Please note that Abilities marked with a superscript are only available in the stated generation or later.
For Generation III and IV games, ignore Hidden Abilities.
Trivia
This Ability's Japanese name literally translates to "reinforced with metal" (which in a metaphoric sense can mean "staunch, hardcore"), explaining why only Steel-type Pokémon have this Ability.
Other languages have similar wordplay:
English Stalwart may contain a pun on Stahl (German for "steel").
French Nerfs d'Acier translates to "nerves of steel".
German Stahlrückgrat translates to "steel backbone".
Italian Volontà di Ferro translates to "iron will".
Spanish Acérrimo means "obstinate", being a pun on acero ("steel").