Lonus is a humanoid, moth-like alien whose four wings and antenna can fold up into a hooded robe (much akin to a poncho), giving him the appearance of a phantom. He has a black body with white patches on his arms, shoulders, legs, the back of his wings, and the back of his neck that resemble ice chunks. Each patch has thick blue outlining except for the ones on his wings, which are outlined in black. There is also a black line that runs on the edge of the outer margin on the back of each wing.
On his torso are three plates: one resembling the chest, a shorter plate under the chest, and one resembling the chest, a shorter plate under the chest, and one resembling the stomach.
On his face are light blue cheeks and underjaw, a mouth that always shows bluish-white teeth, and large cyan eyes with darker cyan spots. The top of his head also sports a blue Y-shaped marking that extends over his scalp. He has three pointed fingers and a thumb on each hand, and two toes on each foot and a third toe-like extension on his ankles.
- Lonus first appeared in Ben 10: Wielder of The Omnitrix, where he failed to stop Ben in the fight in Areans.
His name is Lonus because his home planet is close to the moon.
- Unlike Big Chill, Lonus does not tend to make ice-based puns.
- Lonus was able to become invisible, unlike Big Chill.