After the incident that transforms him, David exists as a semi-gelatinous blue worm of about 12-16 inches in length. In this form he can manipulate his body in various simple ways compressing his shape or length, forming balls or "rope" and so on. He can see and hear but has no sense of taste or smell. When placed inside of a liquid his body camouflages i.e. if placed inside soda, he will take on the same coloration as the soda. He can also easily cling to and move up and down vertical surfaces.
The most prominent ability that David possesses is ability to possess other beings when entering through an orifice i.e. the mouth, nose, ears, vagina, etc.
When he takes possession of someone he is able to gain full physical control of their body. He also has access to their memories which enable to carry on a simple impersonation of the person. He is able to "sleep" and submerge into his host's subconscious which relinquishes control of the body back to the person. Once he has possessed somebody a three step process occurs.
Step 1 - Mental Integration
David absorbs his host's mind and persona allowing him to access them at will. The time it takes to complete is variable. Being "asleep" in someone's body will cause the process to take around 7-8 hours to complete while being awake the whole time could cause it to double.
Step 2 - Physical Integration
David integrates physically with the host's body. This process can also be variable in terms of length of time. Physical Integration technically begins at the same time as Mental Integration but takes longer to complete with a good baseline being around 5 days.
Step 3 - Assimilation
The true (subconsciou) end goal of David's possession. After Mental and Physical Integration he "digests" the host, permanently absorbing their genetic material and making them a part of himself. Assimilation, like Physical Integration, is variable but a solid baseline is around one 5-7 days. During this step, David cannot leave his host.
All told, the entire process from possession to integration to assimilation should take around 10-14 days.
One ability David has, regardless of any level of integration, is the ability split off pieces of his body into pseudopods or "worms" which carry his personality with them and that he's able to control. These pieces all have the same functionality as the "main" body and can take control of others. As with the main body, once they've entered a host the integration/assimilation process begins.
Once Mental Integration is complete, David has full access to his host's mind from their memories (even deeply buried ones) to their habits, disposition and every other facet of their personality. He is also able to alter their short term memories and, with practice, their long term memories as well. At the extreme end, David could completely alter someone's mind changing not just their memories but their entire personality, desires, attractions, and anything else. This requires much will, concentration, and practice with short and long term memories however, and if David isn't careful the effects could be catastrophic both for the person's mind and for his own.
In effect, once Mental Integration is complete the person temporarily exists as a persona within David's consciousness. Should David exit their body then his connection to them is severed and the actual person reemerges in control of themselves. The bits of their mind will also fade from David over a few hours. However if David reenters their body, even after their mind has completely faded, he does not need to go through a process of reintegration.
Physical Integration is something of a percentage meter or timer. If David exits a body for one day, then that's a day lost that would need to be made up to complete physical integration. As said, the time with Physical Integration and and Assimilation is variable depending on a number of factors such as the size of the host and how much of David is inside of them. PI would take longer with Gordon Black than it would with Chelsea Cooper, for example, as Gordon is much larger physically than Chelsea.
Once assimilation has completed the host is now a permanent part of David. Their persona simply exists as a separate persona within himself though one he can swap with. If David assimilated Dana Pak, then he could swap between "David" and "Dana" at will... which effectively means that "David" is simply another persona albeit the main and more dominant one. Likewise an assimilated host ceases to exist physically and the body used is closer to a clone made from the absorbed genetic material of the host.
David, cannot produce an infinite amount of worms and his upper limit should be around ten. He could do a few more but the danger increases as the more bodies he occupies, the more David's consciousness is diffused. Should David try to take over an especially strong or chaotic mind—say Adam Karter—while stretched thin then the result would be his identity being subsumed by the identity he was trying to subjugate. To fix this, David would need to re-gather most of his worms and insert himself into the rogue body to reintegrate it.
David's worms all share his consciousness and so if he's taken over multiple people then they form a sort of hive mind. They can communicate telepathically and if David wanted to he could switch their sensations around. For example, if David was in the bodies of Alyssa Randall, Dana Pak, and Shawn Gregory then he could swap around the sensations of the worm in Shawn's body with Dana's to achieve a pseudo-body swapping phenomenon. This only applies when he's in a body. The pseudo-swap would be rendered null if he exited them.
A fully assimilated body is capable of producing a worm the size of David's original post-transformation body. In this way, David theoretically has a means of infinitely propagating himself should he continuously achieve full integration with his host bodies.
As David's body post-assimilation is just assimilated genetic material, he's also free to alter it in various ways within reason: add or subtract height or weight, change hair, things of that nature. He's also able to use any other absorbed genetic material to further modify the body. This could be from another person he's assimilated or from acquiring genetic material in some other way i.e. through hair, saliva, blood and the like.
From this he could combine features or outright shift his body into a copy of someone else's or perhaps even merge assimilated bodies into one. This comes with a few restrictions, however. Changing a body requires energy and the bigger the change the more "fuel" (via eating) is needed for David to replenish himself. Acquired genetic material likewise only lasts for around a limited time—no more than a few days—and would not bestow any mental attributes of the person it came from, only physical.
This is, of course, different from properly assimilated genetic material.
Besides the previously mentioned restriction regarding how many people David can safely take over at once, another restriction is that he can only take over other humans. On a meta level, this reflects the author's distaste of, and disinterest in, seeing Will become a dog or a cat or whatever other animal. For an in-story reason, the minds of animals are too different from a human for David to properly integrate and control and doing so, even in the most basic level, would cause issues; likely his mind being subsubed by the animals. Instinctively, David will stay away from the idea of taking over an animal. So no animals.
While David, in his normal state, is fairly damage resistant he is not invincible. Acid or other solvents will likely pose a grave danger to him. Exposure to fire or other extremely hot temperatures for an extended period that would likely be enough to destroy him as well. Freezing and shattering could offer a solution but it also might not.
David is most vulnerable during the Assimilation step and if the host is killed then whatever portion of David was inside of them goes with it. After Assimilation, this does not apply and prior to it David can freely leave a body.
In a sense David is a bit like the The Thing, slowly assimilating people into himself and then carrying on as a fascimile of them. He, perhaps genetically, has an instinctive desire to take over and assimilate other people to propagate himself but is still enough of a human that he also, again instinctively, fights against it i.e. he might become restless if he begins staying in a body for too long and feel like he needs to make a switch.
When David leaves a body they have only a vague and hazy recollection of things, as if they were drunk.