It's an anti-paladin. Bad armor, great damage, mage spells.
Basic: Dual Katanas: They can dual wield "main hand" gear.
Mastery: Physical Attack up (217.6%)
Mastery: The Swallows Return: Counterattack Physical Damage (50% cap) -- huge nerf from the 95% in Original.
Starting Class: C. TOO SLOW.
Ending Class: A. The debuffs from magic, teleports, and their pure damage is very appealing. The best Katana is male only (but was made unisex in the 3DS version).
No Class Change: A. Great damage throughout the game. A thief can steal great katanas from some bosses. There are more bosses in the 3DS version that need The Swallows Return to damage them (due to high AC).
They nerfed Swallows Return, a LOT. They cap it out faster but much earlier than before. Range Increase Alchemy is vital as it lets you counterattack things in the back row. Give them beheading too, just for fun.
Fighters and Brawlers might actually out DPS them in the longest term, but Samurais are absurd damage, especially when they get a chance to counterattack.
Amusingly, the best protector (Shield?) is Samurai only in the PSP version.