No enigma here, Catherine, as far as I can see. Your fate (δαίμων) depends on the way you behave (ἦθος). It's not correct, strictly speaking, to say that the sentence has no subject. In a way (since both ἦθος and δαίμων can be seen as predicate nominatives), it has two subjects. Your character is your fate, your fate is your character: is there a difference?
Etymological note that may or may not provide illumination:
Ἦθος, commonly spelled ἔθος, originally meant "habit," "custom," perhaps even "habitation."
Δαίμων literally means "a god," usually a minor deity. It is derived from a verb meaning "allot,", "apportion."