Pretty straight forward question - there are two movements throughout modern philosophy that quite often come together - realism and empiricism. I'm wondering if being realistic must assume being empiric, or can realism defy empiricism? And if so, is there a philosopher who went by this view?
There are many versions of empiricism and realism, but generally speaking, they are rather in tension. Realist positions are optimistic towards knowledge: they assume that knowledge (or at least some aspects of it) are about an external reality, beyond mere experience. Empiricist positions are about the acquisition or justification of knowledge, which they restrict to experience.
For example, some versions of empiricism have it that no inference can be made from the observable to the unobservable. In this sense, realism is incompatible with empiricism. So the answer to your question is: yes, a realist can defy empiricism, and it actually has to do to be a realist, at least this kind of empiricism: the realist claims that our inferences go beyond experience. All realists go by this view.
Weak versions of realism could be compatible with weak versions of empiricism that merely say that knowledge has it source in experience, but that inferences that go beyond are permissible, or that knowledge must be justified by experience but with a permissive understanding of justification. But some versions of realism can still defy even this weak version of empiricism by assuming that we have some direct intuitive access to first principles or to the fundamental nature of things. This view might not be very popular these days but it used to be quite common: it was held by rationalists and platonists, so to go back to your question, all rationalist and Platonist philosophers actually went by this view.
Yes, if you accept realism as the theory (very roughly) (a) that some facts, events, states of affairs exist independently of experience, (b) that these facts, &c., can be known, and (c) that they can be known (or can only be known) conceptually and not through experience.
Descartes comes to mind as a candidate for this view. Experience, or 'phenomena' as Descartes sometimes calls it, provides the material for science but as such it explains nothing. On the contrary, it leaves everything to be explained by principles, namely laws, which are formulated by reason.
A clear statement of this position is Descartes, Principles of Philosophy, 1644, II, art. 64 : 'The only principles which I accept, or require, in physics are those of geometry and pure mathematics; these principles explain all natural phenomena, and enable us to provide quite certain demonstrations regarding them' ('The Philosophical Writings of Descartes, I, tr. J. Cottingham and others, Cambridge, 2009, 247).
We might want to retreat from Descartes' belief in 'quite certain demomstrations' but he does outline the kind of position you asked about.