I don't think you understand Plato. There's a spectrum of purely mouthpiece to actually what Socrates said/thought throughout Plato's dialogues. And generally the earlier the dialogue the more it's actually Socrates, but as Plato's own views developed it became more of a mouthpiece.

Since Republic is middle period, it's definitely not just the views of Socrates, and since it also included the theory of forms it's arguably mostly Plato speaking through the character of Socrates.