One of Locke's most amusing personality quirks is his insistance on being called a treasure hunter, rather than a thief. Take a look:
Locke's descriptive blurb:
"Treasure Hunter and trail-worn traveler, searching the world over for relics of the past..."
Locke: I PREFER the term "Treasure Hunter!"
Old man: Ha! Semantic nonsense!
Locke: There's a HUGE difference!
Terra: You're Locke, right? Edgar told me about you. Is it true you're a thief?
Locke: That's TREASURE HUNTER!
Merchant: You're that thief, Locke, aren't you?
Locke: Hey! Call me a Treasure Hunter, or I'll rip your lungs out!
Right, so, it's something he's kinda particular about, but what's the difference?
Well, a thief steals from everyone, and a treasure hunter "searches the world over for relics of the past." Most people don't see the distinction, especially because Locke's exclusive skill is "steal". His skill level deviates between frustratingly incompetent to superlative. (This is a videogame.) And the stuff he can take is, frankly, not always "relic" quality. And this is Final Fantasy VI, there's already a pretty loose definition.
There's one specific point where all Locke's bluster will really count for something. After the world ends, the city of Narshe is pretty much destroyed, and most of what remains is locked up tight. Lone Wolf, who proclaims himself the world's best pickpocket, greets your party, saying that he couldn't pick all the locks. "Only a Treasure Hunter could do that." Now, this could be the game's hint that you should put Locke in your party. (Well, okay, it's definitely the game's hint that you should put Locke in your party.) When you do, he picks the locks quite easily, with a sly wink and everything. But this isn't an impressive feat of legerdemain. He's just picking locks on doors. The fact that the best items in the game-- Illumina, the Paladin Shield, the Ragnarok esper-- are only available after this little stint of "treasure hunting" is largely irrelevant.
The most obvious explanation for Locke's, uh, nomenclature issues, is that thief is deragatory, and despite the expression, less than honorable. Thieves are worthless brigands, out for money and themselves. Locke's out for adventure. It's important to note that the one comment he made about a particulary piece of treasure being "worth a fortune" is not present in the original script. Locke's not greedy, just nostalgic. And when he does take money from people, I'm sure it's in a "steal from the rich and give to the poor" kind of manner.
But Robin Hood wasn't a treasure hunter. He was a deposed noble standing up to a cruel regime, but there's no real connection to dungeon delving, relic hunting, or, um, anything but the thievery. Locke's a bit more than that.
See, even the game doesn't classify Locke as a thief. Thief is one of the original character classes dating back to the original Final Fantasy. It's made it's appearance in Final Fantasy III, Final Fantasy V, Final Fantasy Tactics, and yeah, Final Fantasy X-2. Every game with a class system, basically. The thief character class has also been applied to people like Rikku and Zidane. But Locke's official, confirmed-with-Final-Fantasy-VI-Advance class is Adventurer. Unlike pretty much any Final Fantasy thief, Locke can use the big damn hero swords. He also gets the daggers and the boomerangs and the steal command, but y'know, so do ninjas.
The whole point of this is that Locke really isn't a thief. He's a treasure hunter. The recent Gameboy Advance translation really hammers this home. He's looking for a legend, and is fifty times more Indiana Jones than Artful Dodger. Thievery might be something he does, occasionally, but the quest is who he is.