In the original series, Kevin is a thin and scrawny boy with shoulder-length black hair, pale skin and dark brown eyes with black-discolored marks around them (a sign that an Osmosian has recently absorbed energy). He wore a torn black T-shirt, black studded arm bands, brown cargo pants, black knee-length shoes, and a padlock necklace which is a spoil from his first crime: stealing a bike and using it to run away to New York. After absorbing an Omnitrix feedback, Kevin was able to transform into exact copies of all of the unlocked aliens, missing an Omnitrix symbol. After the energy overwhelms Kevin, he mutated into Kevin 11, a random mixture of body parts from the 10 said aliens which completely destroyed his clothes, except for his cargo pants.
In Alien Force, Kevin grew into a tall and muscular young man. His eyes now lack the black marks (as his body fully recovered from and stopped absorbing energy). His normal attire is a tight black T-shirt over a grey long sleeve shirt, blue pants, and black combat shoes. His hair, although still long, is styled into a choppy mullet. Occasionally, he is seen wearing a short sleeved light blue oil stained mechanic's jacket over a white muscular shirt, blue pants, and his black combat shoes. After he goes through a second mutation, an unbalanced mixture of various matters, the transformation completely destroys his clothes, except his briefs. As a result, Kevin had to wear an ID Mask in public to appear human. In his second mutation he looked like a mixture of solid materials such as stone, wood, Taydenite, and metal. During Kevin's time as Ultimate Kevin, his appearance is again an unbalanced mixture of various aliens that were currently accessible to Ben through his Ultimatrix.
In Omniverse, Kevin wears the same black shirt but the sleeves and collar are roughly cut, the sleeves on his long sleeved grey shirt are slightly torn and he wears blue wholesters. His hair is longer and straighter. His pants have a small tear below the left knee. He also wears a padlock necklace similar to the one he wore when he was 11 but marked with an "11". He also sports black discolored marks around his eyes, making him appear tired; these marks bear similarity to when he was 11 and still absorbing energy.
Later in the series, Kevin grows a few chin whiskers and his grey shirt gains stripes. In Weapon XI: Part 2, Kevin gets the same cross-shaped scar to his chin that future Kevin has.
12 year old Kevin in Omniverse looks exactly the same as how he does in the original series, except for the fact that the color of his cargo pants are grey instead of brown and his padlock necklace is marked with an "11".
When Kevin was 11, originally he was a criminally insane sociopath, willing to hurt/kill anyone for his own personal gain and/or pleasure. This behavior and mentality stems not only from being seen and treated as a freak by his peers and family, but also because of the side effect of Kevin absorbing energy. His selfishness and sociopath-nature is made apparent when he was once willing to derail a money train to steal the money and kill countless passengers on another train. Kevin had an extremely negative outlook on life because of the countless hardships and struggles he had, feeling insecure about himself as well as having incredibly low self-esteem. He had a deep-rooted hatred for other people and society in general, even the innocent as he once said "Nobody's innocent! They just haven't had the chance to make fun of me yet!".
After being sent to the Null Void and meeting a prisoner named Kwarrl who acted as his surrogate father-figure and mentor in learning how to control his powers, Kevin eventually began to develop a better outlook on life and allowed him to form an actual bond for the first time. This eventually leads to Kevin making amends with his former enemies, Ben Tennyson (His Best Friend) and Gwen Tennyson. As a result, Kevin and Ben no longer share their famous rivalry and instead have become true friends. Kevin acts as an older brother towards Ben. They care genuinely for each other. Kevin is much more open about his feelings, showing gratitude and tries to express it to both Ben and Gwen. He also seemed to have a better connection towards Max.
In addition to his new outlook on life, Kevin showed disgust at those who would take advantage of others such as with Argit when he was taking advantage of an alien's trust. Although Kevin may at times, do things unethically like selling weapons to make money or do things that involve getting money, his heart is always in the right place. He also showed distaste on how others care about only themselves will abandon others for their own selfishness such as when Kevin reprimanded an Alien Prisoner for being selfish in order to save itself while reminding him, about how Kevin himself was selfish before the latter's redemption.
Powers and Abilities
Originally, Kevin's powers were believed to be of an unexplained mutation, then alien heritage but later explained to be of a unique genetic component.
Kevin has the ability to absorb the energy and matter of anything (excluding certain materials) through his body at will and utilize it in various ways. If he's not absorbing excessive amounts of matter or biology directly into his body, he becomes wrapped in a coating that leaves his clothes intact.
After absorbing energy, Kevin can channel energy through his hands to manipulate technology, discharge it as an energy blast or use it to speed up cellular regeneration to increase his strength or recover from injuries at an accelerated rate. However, the energy will dissipate over time or through excessive use, so he is required to recharge.
However, if Kevin absorbs energy, the energy will cause Kevin to become mentally unstable. Storing energy for long periods of time will worsen his instability. This mainly occurs in Osmosians who absorb energy very often but cannot control it due to inexperience and lack of skill. Energy absorption can sometimes easily turn into an addiction.
When Kevin has recently absorbed energy, he develops black marks around his eyes. This is a side effect of absorbing energy.'
Similar to his ability to absorb energy, by absorbing the DNA, metabolism, and life force of other life forms, Kevin can use it as nourishment to sustain his health and strength. Absorbing all of someone's energy will kill them. Kevin can use the energy to acquire the creature's powers and undergoes mutation to accommodate the absorbed abilities.
Despite complete access to any of these absorbed abilities at anytime, they aren't as strong as their pure forms, as noticed by Diamondhead who estimated that they are only about 1/10 their full power, as seen when he was able to catch and crush Kevin 11's crystals with his bare hands while the others shattered upon hitting him. This was proven correct by Aggregor. This is the reason why Aggregor first turned P'andor, Andreas, Bivalvan, Ra'ad and Galapagus into pure energy before he could absorb their powers and abilities so he could attain their full powers and abilities. He also stated that when Osmosians absorb the life force of other beings, the new powers can only be used temporarily unless they were to drain all of that life form's energy.
This ability can also be used to combine the DNA of two beings, which is what Servantis used Kevin for. Kevin was able to absorb some of Servantis' energy that brought out some past memories. Kevin was able to carry the energy around on his hand and used it on Argit and Alan Albright to bring up their past involvement with the Rooters. Kevin then used this energy to his advantage as he was able to permanently lock out Servantis from the Amalgam Kids' minds.
Kevin can absorb solid materials, coating himself in the substance which allows him to take on its properties. He will proportionally increase in physical strength and durability to whatever he absorbed.
Kevin is able to absorb multiple materials at once. He also has the ability to apply to other people a coating of whatever material he has absorbed at the time, temporarily.
Depending on the amount of the absorbed material Kevin has available, and how he distributes it, Kevin can either coat his body, or part of it, in the material as a second skin like armor,, or completely convert his body, or part of it, into a living version of the material. The amount that Kevin requires depends on the density of the material.
This ability has once been used productively when Kevin was forced to synthesize a coating of Taydenite, thus incapacitating him.
In Alien Force, following his second mutation which causes him to lose the ability to revert to his human form, he gradually begins learning how to use his absorbed matter in various new manners. He is able to use high levels of strength and durability as well as regenerative and shape-shifting abilities. After returning to normal, he retained his newfound powers due to him now having knowledge of being able to doing so.
Kevin can repair objects of the current material he has absorbed either by turning it into a liquid form or releasing a liquid version of whatever material he has absorbed. He can partially merge with the actual matter and manipulate the substance's form. Both abilities were used when he repaired the rock ceiling of a mine, fused the seal of P'andor's suit, morphed it around P'andor, and fused the arms together as makeshift handcuffs. He once used the metal he absorbed to create a helmet for Gwen to prevent the Lucubra from controlling her.
Kevin can easily pick locks, using his powers to absorb the material of the lock, then form his finger into a key.
Kevin can also absorb liquids, and when he does he gains the properties of that liquid. If Kevin absorbs mud, objects will pass through him without causing physical damage. He can even absorb bubblegum.
Other Skills and Abilities
Even without his absorption powers, Kevin is shown to be stronger than an average human. Even the powerless Kevin was still a fairly proficient fighter and a useful asset to the team. While not to the same extent as aliens like Tetramands, Kevin is capable of lifting/throwing the combined weight of several people with little strain. He is also capable of resisting impacts and blunt force trauma that would severely injure an ordinary person with moderate discomfort.
Kevin has some noticeable skill in martial arts which he gained during his years as a runaway, a prisoner in Incarcecon, an alien tech dealer, and a member of the Plumbers. With this combat skills, he has proven on several occasions to be a highly formidable fighter, having held his own against and defeated opponents such as the Megacruiser robots, Technorg, and even Vilgax in a one-on-one fight.
Despite lacking a proper normal education, Kevin possesses a vast scientific and technological knowledge of alien paraphernalia. In Deep, Ben and Kevin argue that they both cannot use their powers while being in their Plumber suits. So Kevin made some alterations to the suits, equipping them with propelling units and shock pulses that allow them to attack with electric shocks. Kevin also has great analytical and deductive skills, having been able to determine and find out specifically what happened in a battle between Aggregor and the Plumbers several hours after the battle, greatly impressing his friends and even gaining a compliment from Max that he either could be an expert police officer or a wanted criminal. Kevin has good motor skills, being able to race and chase in his car and the Rust Bucket 3.
He is also great at mechanics and engineering. He is shown to repair his car for the numerous damage it took during the course of the series. He also modified his car, setting alien tech for beneficial gears. On top of that he built the DX Mark 10, a car that he reluctantly gave to Ben as his sixteenth birthday present. Kevin is also shown to be very good with animals, even without trying. In Deep, a Blowfish became strongly attached to Kevin. Later, Khyber's dog, despite being well-trained and loyal to Khyber, ultimately chose to join Kevin, even saving Kevin from being crushed.