Hi there!

So, short answer, Socrates was real. Obviously the most extensive source for this claim is the one you mentioned: The writings of his friend and younger associate Plato. I'll swing back around on this in minute.

Some other important sources for the life of Socrates include things like the works of Aristophanes. In his The Clouds he satirizes Socrates (and through him philosophers more generally) in all sorts of ways. If the testimony of the Apology of Plato is to be believed, Socrates himself blamed this work for giving him a bad reputation that contributed to his being brought up on charges of impiety and corrupting the youth of Athens (the reference at 18c-d to a "writer of comedies" is uniformly believed to be a reference to Aristophanes). Now this work is obviously of quite dubious value regarding the philosophical positions the historical Socrates held, as Socrates is at pains to point out in the Apology, but nevertheless a satire of someone does suggest that they existed.

Other important sources include Xenophon, a soldier, historian, and sometime philosopher who, like Plato, was a pupil of Socrates. Xenophon wrote fairly extensively of his old teacher, including his own account of the trial and death of Socrates. While he and Plato do not agree on every point of detail, the big picture stuff is basically the same. Certianly the accounts are similar enough for present purposes to

Finally, you have tons of references in later classical literature to Socrates which take it as not even up for dispute that there was a historical Socrates. Most prominent among these is perhaps Aristotle, that great pupil of Plato, who in considering the views of his distinguished predecessors singles out Socrates for having put ethics at the center of philosophy. He also elsewhere (in the Rhetoric) remarks, with characteristic acidity, that the sons of Socrates were unremarkable "fools and dullards".

So the upshot is that there was a historical Socrates. We have about as good evidence for that as you could want.

You have, however, hit upon an interesting point, namely that the Socrates we get in the writings of Plato is emphatically not always representative of the historical Socrates. Especially by the time you get to the middle dialogues (e.g. Republic) the historical Socrates has been set aside and the character in the dialogue should not be taken to give any direct testimony regarding the views of the historical Socrates. A good (if very rough and ready) heuristic for this is the presence of the theory of forms/ideas, a metaphysical doctrine that is universally held to be an innovation of Plato's.