 Given the tight connection that Aquinas describes between form and matter, it is hard to characterize his theory of souls in general as  dualistic.  To be sure, in some sense, material substances are a composite of substantial form and prime matter (with accidents on the outside, so to speak, unified only accidentally with the substance). Still, Aquinas takes pains to stress that it is the substance as a whole that properly exists. And he definitely does not think that material substances are a composite of form and body, since the material substance is the body. His unitarian framework deliberately makes it incoherent to speak of the body of a living thing (or of any substance) as something distinct from that substance.