Kevin was 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 shorts, gray knee-length boots, and a padlock necklace which was a spoil from his first crime: stealing a bike and using it to run away to New York. After absorbing an Omnitrix feedback, Kevin becomes able to transform into exact copies of all of the unlocked aliens, missing an Omnitrix symbol and eventually becoming a mutant fusion of all of the Omnitrix's original ten aliens. As a Teenager, Kevin is more muscular and taller. He also wears a black t-shirt with baiged-colored sleeves with blue jeans along with black shoes.
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(His Romantic Interest). 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 (mostly Gwen). Gwen and Kevin continue their romantic relationship. 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.