Locke is a playable character in Final Fantasy VI. He is playable throughout the World of Balance and is required for several storyline segments, and is an optional recruit in the World of Ruin. His skillset aligns with the Thief class, but his actual job title is "Adventurer", reflecting he is a bit more physically adept than Thieves usually are in other titles.
Locke mostly specializes in light equipment. He begins the game with a Dagger, Leather Cap and Leather Armor. His ultimate weapon in the GBA and later versions is the Zwill Crossblade and is won from the Gold Dragon in Dragons' Den. Locke is unusual in that he has an "ultimate weapon" in versions of Final Fantasy VI predating the GBA version, the Valiant Knife.
Due to his unique ability having little offensive utility, Locke's performance in battle is tied to what weapons the player can acquire for him at the time. His access to throwing weapons lets him deal full damage from the back row, and later the Flametongue, Icebrand, and Thunder Blade let him use elemental attacks. Late in the World of Balance, the Hawkeye can be an extremely powerful weapon, and the Assassin's Dagger lets him inflict Instant Death on targets.
In the World of Ruin, Locke's offensive potential is greatly boosted by his exclusive weapons. The Wing Edge gives him great stat boosts and a chance to inflict Instant Death, and the player can acquire two to let him dual-wield. The Gladius, Ragnarok, and Lightbringer are all powerful weapons and the latter also have good stat bonuses, and the Valiant Knife ignores defense.
Locke has an important distinction over other characters in that he is able to equip two weapons that ignore defense; the Valiant Knife and the Ultima Weapon. While the mechanics of these weapons are polar opposite (the Valiant Knife is stronger at low HP, the Ultima Weapon is stronger at high HP), the innately high base power of the Valiant Knife means it can still deal heavy damage at high HP levels. This makes Locke the best character for the Master's Scroll and Genji Glove combo that allows a character to deal eight attacks a turn: with training to his Strength via level up bonuses, Locke can easily hit the damage cap with every strike.
Locke's unique ability is Steal, which allows him to attempt to steal an item from an enemy. The odds of Steal's success are as follows:
If Locke is equipped with a Sneak Ring, the success rate of Steal is doubled.
All enemies have two items to be stolen: a "common" item with a 7/8 chance to be stolen and a "rare" item with a 1/8 chance to be stolen. The game determines what item can be stolen at the moment of Steal, not at the start of the battle. If an enemy does not have a common or rare item to steal when the algorithm determines that is the item that would be stolen, Steal fails.
Equipping Locke with the Brigand's Glove upgrades Steal into Mug, which allows Locke to attack an enemy while attempting to steal an item. The stealing mechanics rate of Mug are the same as Steal. When using Mug, special effects of weapons, like Instant Death, ignoring defense, etc., are ignored. Weapons that have a chance to cast spells when attacking can still trigger the spell, but will do so after the steal attempt and will not count as a steal attempt themselves.
Equipping Locke with a Genji Glove or the Master's Scroll allows him to attempt to steal twice and four times when using Mug, respectively. However, due to a glitch, if Locke successfully steals from multiple enemies at once when using Mug with the Master's Scroll, only the first item will be received and subsequent items are lost and those enemies cannot be stolen from again.
During the Occupation of South Figaro, Locke must disguise himself while moving around town. When battling a Cadet or a Merchant, when a successful Steal attempt is triggered, Locke will also steal his opponent's clothes and put them on. His gameplay while disguised does not change but for his sprite. It is possible for Locke to steal an outfit he is already wearing, in which case the enemy will still be replaced with a Birthday Suit enemy and Locke will act out putting on their outfit.
In the World of Balance, Locke is a required party member from when first introduced to the player in Narshe up to when the player reaches the Returners' Hideout. He has his own segment in the Three Scenarios where he is the primary player character during the Occupation of South Figaro. After the Battle for the Frozen Esper, Locke is available as a party member. Following the player completing the story events of Zozo, Locke is a required party member along with Celes, up until the player completes the Magitek Research Facility. He is available as a party member again afterward until after the Imperial banquet, when he is a required character while traveling to Thamasa.
In the World of Ruin, Locke is recruited by reaching the end of the Phoenix Cave. As the cave is a difficult dungeon that requires two party members to explore, Locke is likely one of the last characters that will be recruited in the World of Ruin. Recruiting him here in the mobile/Steam version earns the More Than King achievement.
When the player can first travel to Kohlingen, if Locke is in the party the player can view two flashback scenes establishing his past with his deceased lover Rachel. These flashbacks are triggered by entering the north-west and north-east houses in the village.
Locke has a few optional dialogue lines that are substituted over the generic dialogue used otherwise when the party meets Ramuh in Zozo. As Locke has the lowest index number of any party member other than Terra, any unique lines he has here will trigger in place of other characters with unique dialogue.
In the World of Ruin, Locke is required to be in the party to unlock the doors in Narshe.