The general consensus is that Judas Iscariot really existed, and he was the one who turned Jesus over to the authorities. Bart Ehrman has a series of posts of his blog where he talks about this very issue. To quote:
>Did he really exist? Or was he “made up” by Christian story tellers who wanted to explain how Jesus ended up in the hands of his enemies? Often it is pointed out that “Judas” sounds like “Jew”: did Christian story tellers come up with the “Jew” who “betrayed Jesus” in order to make a theological point?
I have a definite view about that.
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>I think he was a real person. Actually one of Jesus’ disciples. And the one who betrayed him to the authorities leading to his arrest and crucifixion.
Ehrman then talks about our sources, and what we can say about who Judas was:
>He appears at least to have existed — based on the surprisingly widespread attestation of stories about him and the criterion of dissimilarity. My view is that we can infer a lot from what little historically reliable information is available to us. If we are looking for the bedrock of historical fact about Judas, a critical examination of our sources yields at least three pieces of information: his name was Judas Iscariot; he was one of Jesus’ twelve disciples; and he “betrayed” Jesus by turning him over to the ruling authorities.
This is general view amongst scholars. I imagine the only people you'll find denying Judas' existence are mythicists, who aren't known for their historical literacy.