Slicked Back

Hair that is combed back, away from the forehead, then held in place with a hairstyling product.

Example 1, Example 2

Don't use the trait for the characters that wear their hair in a ponytail. In that case, use i18 instead.
Note, that in some rare instances, it IS possible for a character to have both Slicked Back hair and a ponytail. Case in point: c11286.