Fighters whose personality/characteristics are different than their fighting style

Floyd. Obvious choice here. Listen to him talk, you'd believe he has been knocking people thru the ropes in every fight.

Winky Wright. One of the best defensive fighters in the modern era who was quite conservative at times, he is quite a high-risk player at the crap table. He really is. This dude presses his bets to no avail.

Foreman was the best of both extremes

Young George Foreman - Powerhouse bomber, fairly grumpy and looks angry always outside the ring.
Older George Foreman - Powerhouse bomber, but the nicest guy who found Jesus outside the ring.

Tyson, shy and quiet who had little confidence but in the ring total beast. :)

David Haye , outside the ring he is flash brash and cocky.
I have yet to remember what he is like in the ring as he always cancels his fights.

Mike Tyson is typical, most boxers I've encountered have a persona they use to sell fights, but most of them are humble and down-to-earth outside the ring. In terms of Brits, those I've spoken to include Jamie Moore and Clinton Woods who were both friendly and self-deprecating, but were very competitive in the ring.

The one exception I came across was Anthony Small, who was arrogant, cocky and brash, while in the ring he was ultra cautious and didn't want to get anywhere near a punch coming his way. And then he went all jihad.

Bellew and Johnny Nelson - both are brash and cocky and talk like they're KO merchants, but both are boring to watch.
Whereas Nigel Benn in his early days with his little innocent voice and lisp, yet went out for a "right old tear up!"

Terry Norris was one of the most soft spoken unassuming guys in interviews etc but in the ring he had a chip on his shoulder and went all out to fight, inside and outside some rules

Matthew Saad Muhammad was shy and soft outside the ring, but was a monster inside it.

Pacman, nice guy outside the ring. Explosive inside the ring.

Joe Louis. Beloved outside the ring. Feared inside it.

Luis Ortiz. Humble and articulate outside of ring. Savage inside.

Amir Khan. Talkative and irritating outside the ring. Unconscious in the ring.

Remember when John Ruiz would run full throttle to the ring only to grab a guy and hold on. Thurman talks like a live action cartoon character about all kos but has proven he can coast and stink it out.

How about the Pakistani guy who showed all the dance moves on his way to the ring and glared at his opponent during the instructions?
Up to that point, he looked like the second coming of Prince Naseem.

Bernard Hopkins. Early in his career he played up a street thug/ex-con image and then would get in the ring and put on a defensive boxing display (Tito and after). Later on, he would play the "I'm old but not a normal human" routine and psychologically beat many younger fighters.

Evander Holyfield is the ultimate anomaly for me though. He would pray and never say anything bad about his opponent and then get in the ring, abandon all of his (impressive) skill, and tear into the other guy with everything he had (yes, even his head). He was so damn competitive and aggressive that he was notorious for continuing to fight after the bell.

Oscar/Ray Leonard are my last choices. They both had that same quality where they looked like pretty boys who always smiled but had a formidable mean streak they hid from the public. Look at Leonard punching Donny Lalond in the THROAT! Look at the glee Oscar takes in brutalizing hated rivals like Vargas, Mayorga, Hernandez, JCC (2nd fight), and Ruelas. Both were much tougher and meaner than one would ever anticipate if only going off how they looked/appeared.

One more I just thought of: Vinny Paz. Watch him during his prime in a room or standing next to Evander, Sweet Pea, and Meldrick Taylor and you would think he was the best of that bunch by far. Watch them all in the ring and you would see Paz was so much worse, it didn't even look like he was in the same sport.