Prime is actually a young teen-ager whose Ultra-Power allows him to create a "goo" that encases him, changing not only his appearance into that of an extremely muscular grown man, but giving him numerous super-human powers as well. He retains the thoughts, memories and consciousness of his young self as Prime.
When transforming into Prime, liquid flesh projects from his torso. When changing back the goo melts off. The change back happens either when Kevin wills it or when he tires. Once depowered, he must crawl out of the blob or be suffocated.
Superhuman strength, stamina, durability, and flight. He also has the ability to concentrate and release energy as a concussive blast. All of his powers are manifested by his own consciousness so his powers are limited only by how strong he thinks he should be. He also has assumed other forms such as Space Prime, Rogue Prime, Final Prime, and Spider Prime (the form he became while helping Spider-Man fight the Lizard in the Marvel Universe).