XLR8 is the Codon Stream's DNA sample of a Kineceleran from the planet Kinet.
XLR8 resembles a semi-armored velociraptor. XLR8 has wheel-shaped feet and wears a helmet with a visor, leaving the other features of his head unknown. Whenever that visor does come up, you can see that he has a blue face, green eyes and black lips and stripes. XLR8 wears the Omnitrix/Ultimatrix symbol on his chest.
In Ultimate Alien and Heroes United, XLR8 looks the same.
In Omniverse, 16-year-old Ben's appearance is the same as Ultimate Alien, but taller, and wears a green shirt like Diamondhead's, has 4 blue stripes on his tail, and is more muscular. His claws are also longer. 11-year-old Ben as XLR8 retains the same appearance from the original series, but his whole neck is white and his tail has 3 blue stripes instead of 5 and is shorter and the Omnitrix symbol is now green.