As far as I'm aware, there is no credible defence of the view that Socrates did not exist as a historical figure. The amount of evidence we have for his existence is well attested to. Most prominently, Socrates is featured as a character in the "Socratic dialogues" and Aristophanes' plays.
It's worth noting that while Plato and Xenophon are the most famous authors of Socratic dialogues, they are not the only (nor likely the first). At the very least, there were 7 of Socrates' contemporaries who wrote Socratic dialogues. Gregory Vlastos is perhaps the most famous contemporary Socratic scholar; his book, Socrates: Ironist and Moral Philosopher is a great source for information of this kind. Another great source of historical information on Socrates and contemporaneous figures is Debra Nails' The People of Plato.
While there is no (credible) doubt that Socrates existed, there is a great deal of debate about what he was like. The problem of characterising the historical Socrates, and differentiating him from the character who appears in various dialogues, most notably Plato's, is often called the Socratic Problem. A good article on it is available here. If you're interested in that, I could go into more detail, but as it isn't the question you originally posted, I'll end this here.
hi! you may be interested in these previous threads
What historical evidence for the life of Socrates do we have outside of Plato and Aristophanes? (this post is referenced/quoted by /u/ADefiniteDescription), and this follow-on post What historical proofs do we have of Socrates' existance?
What can prove the existence of a human being in the far past excluding their body; e.g. for historical personalities such as Jesus and Socrates? - regarding methodology
tangentially related: