Advisor
The Combine Advisors are a race in the Combine to which the posthuman's are enthralled to. They seem to direct operations on behalf of the Combine (if they are not in direct control of it themselves).
The advisors in Half-Life 2: Episode Two
In Half-Life 2: Episode Two the advisors appear "in the flesh".
Abilities
The Advisors have the ability to manipulate objects in telekinetic way. They have been observed to levitate (they have no obvious locomotive limbs), crush metal barrels and human corpses, lift and throw objects and even tear open buildings.
They also seem to have an, at least, basic grasp of telepathy and are able to induce mental pain in humans.
Equipment
Combine Advisors are sheathed in a sort of jumpsuit. They have apparently become so dependant on their technology that they would be unable to survive without it. Mounted on the jumpsuits are a pair of arms to allow the Advisors to have some degree of fine motor control. This implies that their telekinetic abilities are limited in ways that necesitate direct manipulation. The machinery on the suits was once speculated to provide antigravity functionality. This may not be the case, considering the Advisor's telekinetic ability.
Proboscii
At least some Advisors have a prehensile proboscus. This has been seen to be used to penetrate human skulls with a stabbing motion. The method was as deleberate (twice witnessed) that it implies intent. It seems the most likely explanation is that they use this to read memories from humans.