The protagonist of the Euthyphro (as well as all of Plato's other dialogues). Socrates seems to be a very simple man, not having many material possessions and speaking in a plain, conversational manner. However, this apparent plainness is all a part of the irony characteristic of Socrates' method. Professing his own ignorance, he would engage in conversation with someone claiming to be an expert, usually in ethical matters. By asking simple questions, Socrates would gradually reveal that his interlocutor was in fact very confused and did not know anything clear about the matters about which he claimed to be an expert. The quest for wisdom and the instruction of others through dialogue and inquiry were considered by Socrates to be the highest aims in life: one of his most famous sayings is that "the unexamined life is not worth living." Socrates himself arguably never advanced any theories of his own, and certainly many of the doctrines that appear in the later dialogues are of Plato's invention. In early dialogues, such as the Euthyphro, Plato presents us with a Socrates less informed by Platonic philosophy; he is, rather, more of a foil for his interlocutors who claim to have positive knowledge.
The interlocutor of the dialogue, and its namesake. Euthyphro is an orthodox and dogmatically religious man, believing he knows everything there is to know about holy matters. He often makes prophecies to others, and has brought his father to trial on a questionable murder charge. We do not know whether or not Euthyphro is a historical personage or whether he is a fictitious invention of Plato's.
The man chiefly responsible for pressing charges against Socrates, bringing him to trial, and having him executed. Little is known about Meletus and by all accounts, he seems to have been a rather insignificant figure. Plato's portrayal of him, both in The Apology and in the Euthyphro, is far from sympathetic; Socrates' cross-examination of him in The Apology puts him to shame. He does not actually appear in the Euthyphro, but he is mentioned on a number of occasions.