Backstory time.
This fellow appeared in one of my many uncanny dreams. This guy didn't belong to any tamer in my dream, I simply saw the mon do a few interactions.
such as...
* Getting shunned by a pack of Gazimon (which this digimon originates) because of his color and "wrong" tail.
* Being chased away by some Gazimon again because of his skull both makes this digimon a scaredy-cat and "you think you look so cool in that don't you?".
* And chased by a Gryzmon/grizzlymon for unknown reason, SkullOkamimon throwing shadowy orbs towards the front of themselves which vanish after a moment to reappear near grizzlymon for a weak hit or distraction.

Yup that's the poor guy. ^^;
Other moments in my dream had them sulking or pouting, likely due to all that treatment.
They could be more brave if the digimon had a trainer or friends that weren't scared or jealous.

Thier core ability is Vanishing Strike, teleporting a projectile or any object already midair to some other location for cost of weakening the impact or secondary effects. They usually use this with a borrowed ability of shadow ball mentioned in my dream glimpses. I'm sure if they were brave enough, they could melee with the shadow ball and have something else strike with the main ability to ensure proper or better hits.

Now the skull typically covers entire head. For a different SkullOkamimon, it might have a gap in the back that exposes Gazimon-like ears which are bundled in a ponytail (that was too neat not to include RayCovert hehe).
The skull moves as if it was part of thier body, like one of the many skull head demon-ish characters out there.
Yet the skull also simply acts like the skull of a Cubone pokemon, "just" a mask/helmet.
As far as I know, the skull cannot be removed/separated, but it CAN open wide enough to expose a normal-ish Gazimon-like face underneath.

Art by  ReyCovert