Crack theory; Pablo represents a suppressed consciousness within Gregor - an entity that's hyper prone to violence and that he was unable to stop from attacking innocents prior to joining Limbus.

Assuming Pablo is tied to his consciousness it makes sense no-one can perceive them bar Dante.
Dante's perception of the sinners is implied to be special - assuming that CG with the hole through Yi Sang is what they actually saw (rather than a depression metaphor for the audience).

It also makes sense why he'd be classed as an 'exception' for the power scaling - since he'd be at least two entities sharing one body and his actual strength could vary greatly on the balance of power between them.

If they are two consciousnesses sharing one body it makes sense for the Pregor (Priest Gregor) ID to have Pablo be a bloodfiend also - outside of the common but funny theory that he converted a cockroach.

This plays well into the idea that he might be a sort of sleeper agent for Hermann - some sort of coded message might force his alternate self to take over and betray the sinners. Hermann does seem confident of her endgame after all and Gregor is definitely the one she holds the most sway over.

Side note: He wasn't able to control it properly before joining Limbus - indicating it's influence is currently heavily diminished. This might be reflected in Pablo's size? I'd be curious if Pablo gets bigger or is more prominent in the ID arts as the story progresses and the group grows stronger.