The phrase "Yūsha Satan yo kiseki o okose!" is translated as "Make a Miracle, Satan the Hero!" or "Valiant Mr. Satan, Work a Miracle!" . Why is there a yo particle after Satan and not a ga particle? Wasnt yo supposed to be an ending particle for emphasis? What does it mean here?
I believe this is the vocative よ, similar in meaning and usage to the English O. EDict mentions this meaning:
(2) (after a noun) used when calling out to someone
恋人よ、我に帰れ。Lover, come back to me.
Japanese Wikipedia has this example from Latin:
Quo vadis, domine?
You could translate the sentence as "O Satan the Hero, make a miracle happen!"