In his changed state, David weighs only a few pounds and is only about the size of a human arm. His body is semi-gelatinous, and he can shape it into balls and ropes, but he cannot take on any complicated forms. He can extend pseudopods from his body and can move swiftly along in a snake-like manner; he can cling easily to and move up or down vertical surfaces. He can compress his body to fit into small spaces. He is an iridescent blue in color when exposed to air, but when immersed in a liquid his body takes on the coloration of the liquid. (So, if dropped into water, he will become translucent.) In his natural state he can "see" and "hear," but he has only a rudimentary sense of touch (he is not very sensitive to temperatures) and he has no sense of smell or taste.
He can possess other people by moving into orifices containing soft tissues: mouth, nose, anus, vagina. He exit through those or through a penis.
When he is inside a person he has complete control of their physical body. He also has a crude grasp of their memories, so that he can "fake" being that person. However, he can only assume a more full control of their mind (gaining not only their memories but their habits and dispositions) after immersing himself in their subconscious. By doing so, he loses control of their body and access to their senses; the person he is possessing re-emerges. However, after he has dipped into their subconscious mind (and spent a few hours in it) he is able to carry on a full and impeccable imitation of them. He can be swayed by their personality, so that he will find himself attracted to or repelled by what they like and dislike, but he is able to override these dispositions when he wants. He can also alter his hosts' short-term memories, by erasing or rearranging them, or giving them new, false memories.
He can at any time sink back down into their subconscious mind and let the original person take control of themselves; he can do this, for instance, when he has no interest in pretending to be that person for a little while.
He can split himself into multiple parts and put these pieces of himself into other people. In this way he can fully control more than one person at a time. The experience, which is disorienting the first time he does it, is akin to having extra limbs. Once he has fully digested a host's mind, he can easily carry on fake but natural-sounding conversations between his host bodies, thereby fooling observers into thinking that these hosts are independent beings.
He can only separate himself into twenty pieces, however. These "worms" are still linked to his consciousness even when they are outside of host bodies, and he can send them on missions and observe the world through them.
When he is in multiple people, he can rearrange their motor and sensory inputs and outputs, so that different minds can look out of and control different bodies. He can't actually put one mind inside another body, but the effect is the same. If he releases his control of people who have switched this way, his victims will have the impression that they have swapped bodies. David can, if he elects, let his victims dip into the memories of the bodies that they find themselves in. For example, if he has put himself inside Adrian, Brandon, and Chris, he can switch the inputs and outputs in Adrian and Brandon, giving them the impression that they have switched bodies. He can give Adrian access to Brandon's memories and personality, letting Adrian imitate Brandon perfectly, while withholding all information about Adrian from Brandon. As Chris, he can watch the resulting fun.
Two special restrictions:
(1) If David is inside a body that is itself changed by exposure to SX-2, then the newly changed body does not have the ability to possess multiple people, only one at a time.
(2) Adam Karter is one person who is semi-immune to David's power. Though he may not instantly recognize that he has been possessed, he will eventually recognize this fact, and can turn around and take over David's consciousness. In effect, Adam becomes the main character (he is thinking David's thoughts for him). Adam's use of David's power is more limited. He can do multiple possessions, but it is much harder for him to run separate people. Also, he cannot fully expel David's body from his own or else the connection is broken and David reemerges.