The Rogue can be many things, but if asked for a generalization anyone could answer 'thief'. However this does not limit it to just that, for that is just generalization. The kleptomaniac who has a keen interest in how to open things? A rogue. The headstrong ex-warrior whom favors stealth and somewhat-dirty-but-clever tactics in battle? A rogue. Interpretation is open to the mind.
Rogues need to be careful; their defensive gimmicks do not benefit from DEF/RES as much as they do from CEL/GUI (In order to use Evasion, Southern Wind and Certain Traps to it's full potential), and that makes them quite fragile.