The human-alien hybrid who goes by the name Ashlar Doxon has a biology and abilities that combine those of both humans and Gallifreyan Time Lords, the humanoid species that forms half of his "parentage." A medical professional examining him would categorize him as human, but with some unusual congenital deformities. His heart is relatively weak, its beat shallow and his blood pressure quite high; in the right side of his chest cavity, a tangle of blood vessels form a rudimentary, unfunctioning pump. His brain structure exhibits some abnormalities, as well, though the examiner would be quick to notice that Ashlar's mental faculties are in no way impaired. In fact, he's in the very upper range of human intelligence, with a remarkable (though not eidetic) memory, an ear for languages (he's fluent in hundreds, both past and present, including many of non-Earth origin), and a genius for mathematics, science, and engineering. He can perform calculations in his head that would be beyond the capacity of even most savants, and code-switch between languages, both written and spoken, mid-sentence without pause. He can, however, forget things and make mistakes in his calculations and language memory—he can never predict these errors, and they frustrate him enormously.

His eyesight and hearing are perfect, and his body is unscarred, except for a smooth white scar at the base of his sternum. Much of the wear and tear that a 39-year-old body should have experienced, his body does not exhibit. He ages, but lightly—he will always look young for his age, and may, if his heart doesn't give out, live to the maximum human lifespan, beyond 100 years. He heals with the quickness of a child, and rebounds quickly, though within human norms, from injury. He has a slightly greater resistance to radiation poisoning and to other poisons than most human beings (though, conversely, he has strong allergic reactions to some common substances, including aspirin and, much to his un-amusement, celery).

The drums, the 1-2-3-4 psychic signature unique to the Master, did not carry over to the metacrisis—though he does have above-average psychic sensitivity, for a human being. With skin-to-skin contact and concentration, he may be able to catch the barest hint of what someone else is thinking or feeling. Similarly, he has a rudimentary time sense—just enough that disturbances in space-time near him may put him on edge, though he may not be able to pinpoint why.

He's practiced with edged weapons (rapiers, broadswords, Japanese-style swords) and concealable weapons (knives, poisons, small firearms), and usually keeps a discreet, well-made handgun on his person—he's engineered a small disruptor chip that keeps it from registering to metal detectors, though someone patting him down will find it.

The "child" of two parents, one a near-immortal, sociopathic, megalomaniacal, obsessive alien and one a very human woman, repressed, loyal, cruel and detachedly curious and cold, needing approval and permission, drawn to powerful personalities, the metacrisis who calls himself Ashlar Doxon has a complicated and contradictory psyche. Pieced together from the memories of the Master and the emotional memories of Lucy Cole, he struggles with bridging the gap between his two contributing personas.

Ashlar believes himself inherently superior to anyone and everyone else on Earth—he is the pinnacle of (half-)human intelligence, attractiveness, charm, and ability. No one can touch him, and he enjoys placing himself in tense situations—standoffs and fights on the edge of breaking out—and breaking them down. He doesn't fight, not physically, and will often evade even verbal arguments, but he has the politician's—or the cultured hostess's—way of talking around difficult issues. He can make people comfortable; he can make them feel silly about behaving poorly. He knows how to flatter, and how to appear to serve while really controlling a situation and working towards his own interests.

He goes to great lengths to avoid pain and violence, but, when they find him, he enjoys them, feeding on his own fear. Pain is arousing, as is being held helpless, watching whoever's holding him captive make the choice to push ahead or to pull back and let him go. This concerns him; it conflicts with parts of his personality that tell him his person is sacrosanct, so it also has the added edge and thrill of personal taboo.

He can be reckless, but he can pull back from recklessness at any time—going from a state on the edge of hysteria to cold, impersonal detachment in the blink of an eye.

Luxury is his by right, and he claims it easily. Cleanliness is next to godliness. Everything he owns is perfect and pristine, engineered to a T, top-of-the-line, both beautiful and functional. He does not, however, care for things that are showy or ostentatious in a landed-family, old-money, leather-and-damask fashion. He takes pleasure in the new, the sharp-edged, and the streamlined—and in seeing the old burn.

Relationships are difficult for him, and he avoids forming any lasting attachments. He's drawn to people in power, particularly iconoclasts, and has formed liaisons with several of his clients—a tendency he sees as a weakness. The relationships tend to be entirely sexual; he does not invite emotional connection, and does not let these flings color his professional dealings in any way.

Victims earn his contempt. He will kill them or save them, with the former being the act he considers the greater mercy.