This character was created using the Eberron Campaign Setting (race and feat) and The Player’s Guide to Eberron (a feat). You could substitute the Races of Eberron book instead of the ECS.
An ex-gladiator Hardflesh offers his services as muscle to whoever pays him; he is usually hired to kill people but he’s not picky about the work. His effectiveness and peculiar brand of honor has garnered him some reputation within the underworld. Alternatively Hardflesh could be a player character; following his own code of honor as he seeks to avenge himself against those who wronged him and overcome his bloody past.
See the discussion tab (above) for more information about the character's development and some balance issues if you allow him as a player character.

Stormreach features a small arena on the outskirts of town. No one knows what the ruin was originally used for but the structure, featuring a number of giant-sized stepped levels running up from a low central area has often featured gladiatorial combat between desperate slaves and horrible monsters from Xen'drik.
What the arena observers didn't realize is that often the winning slave was really one individual, the changing called Hardflesh. The operator of these games realized fairly quickly that slaves, especially ones that can fight, are hard to come by. Having someone he could throw into the fights to spice things up, and who could be guaranteed to execute someone in a particularly bloody fashion when the crowd was getting bored, has been essential to maintaining attendance. Whole events could be scripted knowing that a generally incompetent slave who was acquiring popularity could be assured to vanquish three opponents of greater skill. The changeling who calls himself Hardflesh made this possible.
Pulled from the streets of Stormreach and raised by the Arena owner the Changeling serverd faithfully as the main attraction for more than a decade. A year ago he came to fully appreciate his value and demanded partnership in the arena. The operator saw things differently, and arranged for several of his most vicious employees to teach the the changeling a lesson. Concerned that the gladiator would be too dangerous armed the operator arranged for the changeling to be ambushed while unarmed.
The enforcers were found a few hours later; torn limb from limb.

The operator has increased security, but he still sleeps poorly at night.

Hardflesh (often uses pseudonyms) CR 5
Male Changling Fighter 4/Warshaper 1
Lawful Evil Humanoid (shapechanger)
Init +2; Senses Listen +0, Spot +0
Languages Common

AC 18, touch 12, flat-footed 16 (+4 Armor, +2 Dex, +2 Natural)
hp 47 (4D10+1D8+10);
Resist Immune to Stunning/Critical Hits, +2 vs Charm, Sleep
Fort +6, Ref +3, Will +0

Spd 30 ft. (6 squares)
Melee Claw +14 (1D6+7)
or Claw +12/+12 (1D6+5)
Base Atk +4; Grp +8
Space 5 ft.; Reach 5 ft.
Atk Options Power Attack
Combat Gear None
Special Actions Cleave

Abilities Str 18, Dex 24, Con 14, Int 12, Wis 9, Cha 10
SQ Changing Traits (Minor Shapechanging, Natural Linguist, +2 to saves vs. charm/sleep, +2 to bluff/intimidate/sense motive), grow natural weapons, immune to stunning and critical hits
Feats : Cleave, Educated (Disguise & Bluff as class skills), Power Attack, Perfect Reflection (Players Guide to Eberron), Weapon Focus (Natural Weapons)
Skills Skills: Bluff +10, Climb +5, Disguise +18+perfect reflection bonus, Intimidate +10, Jump +5, Sense Motive +1 (untrained)
Possessions Amulet of Natural Armor +2, Amulet of Mighty Fists +1, shiftweave outfit, Mithral Chain Shirt, (forged) Identification Papers

Eberron is a low powered game world; having more than one level or a level in a PC class is rare. A character with both is truly remarkable. In Eberron Hardflesh is a rare individual but in worlds where higher powered NPCs are common it may be different.
For example in a world like the Forgotten Realms a changeling pit fighter with more than a decade of experience could easily be Fighter 9/Warshaper 5.
A rogue version would take longer to get into Warshaper but is possible. A better idea from a mechanical basis would be use a variant fighter from Unearthed Arcana who has exchanged his bonus feats for sneak attack damage. Preserving full BAB progression would insure the eariest possible entry into the class. The thug class seems to be a poor choice to me, as an experienced gladiator would normally have proficiency with martial weapons and heavier armors.
A more powerful and experienced Hardflesh could specialize in killing a specific, dangerous kind of foe such as arcanists.
I don't think its necessary to use some sort of gladiator class, "regular" fighter works fine to my mind. But someone that would be another variant possibility.

Years in the arena, fighting as hundreds of men and women in all manner of situations, has given Hardflesh the sort of small scale combat abilities possessed by few living beings on Eberron. He has known betrayal and assumes the worst of people. He is completely at home with violence and prefers fighting to talking; there is little he would not fight for the right price. He has tried his hand at more legal professions including as a bounty hunter and a body guard but he lacks the skills; so he offers his services on the open market and, for the few who can pay his prices, he is a reliable and adaptable agent. He is not particularly greedy, but he sees money as being synonymous with respect; something he craves. Attempts to cheat him will be taken extremely personally and he reacts coldly (and possibly violently) to joking or teasing he perceives as being directed at him. Hardflesh has only a rudimentary awareness of other Changelings and their culture. He takes pains to avoid revealing he is a changing to others. He would identify most with the Becomer mindset, shifting forms frequently and at a whim; though never, if he can avoid it, when he thinks he is being observed.

Hardflesh is very dangerous against lightly armored foes; doubly so if the target is a caster who prefers to fight at a distance and he can use his shapechanging abilities to catch them unawares. He has difficulty against heavily armored foes and even DR 10 will largely neuter him. Likewise his armor class and hit points are low; so he will not last long against a skilled combatant of the same level in a straight fight.
His shapechanging prowess is a point of pride, but so is his adaptability, so while he prefers using his natural weapons he will not hesitate to take advantage of a weapon if the situation demands it.
The Changeling knows little of magic and may underestimate what a skilled arcane or divine caster is capable of. Likewise he places a great deal of confidence in his shapechanging abilities to protect his identity and misdirect retaliatory attacks.
If he knows he will be facing someone with spellcasting powers or else in an area where a struggle could be overheard he will try to acquire a Potion of Silence (the spell will be centered on him).
If he is worried his target is particularly dangerous or will be wearing heavy armor he will try to acquire (and use immediately prior to the attack) a Potion of Bull’s Strength.
Hardflesh is not suicidal, and he's been on the loosing side of enough fights in the arena to know better that to stick out a losing battle; he won't stick around things go against him.

If he has been employed to kill someone Hardflesh will seek to approach his targets unawares and strike suddenly. Generally he will observe a the target for a day or so, identify a time when the person would be vulnerable and seek someone of little combat ability who could reasonably approach the target unawares. This person could be a maid at an inn, a servant, an advisor or someone else the changeling thinks he can attack and render unconscious quickly. Usually he will attack and attack using non-lethal damage; once they are subdued he will leisurely examine them, taking any necessary equipment they have that would aid in his deception and duplicating their appearance. If Hardflesh uses his Minor Shapechange to Mimic that person when in their presence Perfect Reflection grants him an additional +6 to his disguise bonus (seeing through this disguise with a spot check would require success on an opposed spot roll against his modified bonus of +24). If he has clothing and other equipment from that person I would be inclined to add a +2 circumstance bonus to Hardflesh as well. If he can’t find someone he will probably just pose as a messenger and try to bluff his way in with a large trunk (a small item can be passed off at the door, but a larger item can reasonable be carried inside).
As a changeling’s Minor Shapechange ability does not register under detect magic a cursory inspection will that spell or Arcane Sight will not give him away but more intense probing (detect thoughts, etc.) certainly will.
He will invariably be carrying a set of (forged) Identification papers for his “escape” identity; the escape identity will be something reasonable, like a Lamplighter checking lamps in a darkened part of town, a drunken minstrel, and assistant to a minor noble family from the area on a business errand, etc. Hardflesh is not a thief any items he steals from a companion of the target to help in impersonating them are likely to be discarded quickly.

Hardflesh is hardly a master strategist; smart players should be able to out think him.