In one of the Thing-Thing series' most notable features, Project 154's physical appearance is determined entirely by the player at the start of each game. His basic body structure is still the same as everyone else in the Thing-Thing universe: a perfectly circular head with no facial features other than 2 eyes, a perfectly circular featureless torso, two hands, and two feet, with no interconnecting arms, legs, or neck.
The story of Thing-Thing follows #154 as he awakens in containment cell #318 at Research Building A: Genetics Division. According to internal retroactive continuity, 154 forcibly kills by hand a doctor who had arrived to administer muscle enhancers and behavioral suppressents, seizes his victim's handgun and shoots the ceiling vent off of his cell. He proceeds to kill the four guards in the service hallway, slaughtering his way to the surface. However, he is halted by Poseidon, an Alpha-Class Bio-Android created by the facility, and assigned to prevent escape--a short-lived affair, as 154 quickly puts down Poseidon and makes his exit into open air. During his escape he learns of the identity of the faction that held him--Systems Corp., a large megacorporation with the slogan "Innovating Life", hinting at their involvement with genetic engineering--and that they deployed the other two elite bio-weapons developed by the facility, Zeus and Hades to hunt him down.
Ostensibly, this is where the story of Thing-Thing 2 begins. 154 wanders into a city where he encounters countless Systems Corp. mercenaries who, though mostly unarmed, endeavor to stop 154 by force. He escapes the city, wandering into a desert inhabited by a horde of undead mummies. Fighting his way through the sand and towards an industrial sector, he eventually meets with Zeus, one of the elite bio-weapons deployed to hunt him down. Destroying Zeus, 154 seemingly goes into hiding.
154 is still in self-imposed exile by the beginning of Thing-Thing 3 when he receives a communication from an anonymous individual who is aware of his origins. Following instructions, 154 fights his way through Sunset City, a housing development for SystemsCorp employees. Arriving at the agreed location, 154 meets his strange contact--in truth, Project 153, developed as a sort of omniscient mobile armor pilot, who reveals that 154 is a form of advanced infantry designed to be resistant to the hydrostatic shock of enemy bullets. The two clash, resulting in the destruction of 153's mobile armor, though he survives. He casts 154 into a secret Systems Corp. service tunnel, encrusted with ice and filled with more mercenaries. 154 manages to fight his way back towards the top, only to be cornered by 153 again, who has not only trapped 154 in a biowaste disposal facility but has also revived an unstable bio-mech from within--the Hades prototype, Vahl Dreig. Despite this daunting task, 154 makes short work of the prototype, only to be reminded that he is still trapped in the containment chamber.
Thing-Thing 4 starts only hours later, recapping the events of the three previous games through animated cutscenes before 154 retrieves the energy core that powered Vahl Dreig, commenting on the volatile nature of such cores and how Systems Corp. stopped using them for that reason, before tossing the device to destroy the steel-reinforced walls. He escapes into the incinerators, where he fights his way through legions of trained Systems Corp. soldiers. He eventually makes his way into the offices and, having resolved to kill off the CEO of Systems Corp. as revenge for his creation, storms into the management branch of the building. However, there he is confronted by 153 again, this time with a small army at his back. 154 escapes the building by jumping out of a window, landing only to find 153 in pursuit piloting a modified Mi-24 Hind-A helicopter. However, 154 commandeers a nearby vehicle, using a Heckler & Koch HK21 light machine gun found within to shoot out 153's fuel tank and destroy the helicopter. 153 is presumed dead in the crash, as so 154 supposes that 153 was killed on impact of the helicopter.
154 travels to Systems Corp.'s genetics research laboratory in hopes of finding the CEO. There, he fights through yet more soldiers, in the process learning of experimental Systems Corp. weapon systems, and the fact that Systems Corp. is now dealing in undead and synthetic lifeforms, as well as finding the very tube he was created in, now empty and covered with warning tape alongside 153's, and the tubes of Projects 110 through 152. Fighting through a heavily-defended quarantine elevator, 154 finally encounters Hades in the form of a disheveled and insomniacal man, capable only of weak melee attacks but with incredible physical endurance. However, once seemingly dead, Hades transforms into a large spider-like bio-mechanical being, armed with the powerful Hades Rifle--a slow-firing but powerful weapon whose shots can penetrate multiple targets at once. 154 kills Hades, taking his weapon and storming the final room. However, there he encounters only countless screens bearing the image of the smug, cigar-chomping CEO he came to kill, who sends an endless legion of soldiers to confront 154 after revealing that he is but one clone in an army of millions. And then the CEO then gave a speech to 154, telling why he created him, and the purposes and intents they he wished for once 154 is created. But then he tells 154 that he is, at least, the most powerful bio-weapon on Earth, but just that he's too strong for the company to control. And then the CEO bids farewell to 154, and dispatches the army of soldiers to 154, along with deploying a gas that disables 154's healing ability. There, 154 is apparently shot dead and his corpse used to refine the genetic splicing process.
Project 154 is an extremely expensive bio-engineered soldier, and as such is an extremely deadly warrior. He possesses enhanced strength, extraordinary stamina, and great dexterity and agility. He is very proficient and deadly accurate in a wide array of firearms, and is no less deadly in hand-to-hand combat. He appears to have a high level level of driving training and/or experience, enough to drive a sports car down a city street, dodging attacks from a helicopter while simultaneously returning fire. Also, as a result of his bio-engineering, he can withstand powerful blows and even bullets without showing any outward signs of pain. In the second and fourth games he also possesses an accelerated healing factor, although this may be a gameplay design choice by Diseased Productions rather than a canonical ability. However, the offical guide of TT4 states that said healing factor is one of the benefits of being a super-soldier, making it likely that the ability is canonical.