punkofnice,
As a secular Jew, there is actually some reliable historical Jewish documents that help the secular to formulate a picture of the rabbi that people call "Jesus of Nazareth."
Jesus "Christ" is someone different, and you are correct that such a character as that which is described in the Gospels likely did not exist.
That figure was loosely based on the Jewish Messiah, a concept that was both post-Biblical and fluid, one that kind of crashed after the Bar Kochba revolt which began in 135 CE (and, as we all know ended in disaster). The Jews still kind of talk about the coming Messiah, but for the most part after Bar Kochba, Jews generally no longer believe it refers to the coming of a personal monarch.
The rise of the Jesus movement from the Jesus "character" has a lot to do with the history behind the rise of the Bar Kochba revolt. Christianity did not rise in a vacuum. It was a real sect of Judaism, and it arose because of a real rabbi who either claimed to be the Messiah or whose followers believed or hoped was the Messiah.
While Jesus is spoken of in rabbinic texts, written as early as the first century, they appear edited as late as the third. Whatever they said or didn't say before, they were altered to make references to the written gospel claims to things such as the virgin birth and being the "Son of God," etc. These additions may be minuscule, but do make them somewhat questionable (and funny). You see the rabbis not wanting to mention Jesus at all, but then suddenly say something like: "By the way, that Jesus fellow, his mother was a whore." And then move on to a totally different subject. Very weird. (for example, Lev. Rabbah 14.5) Some of the comments are older, some newer. It's complex.
You can tell if the comments are new by what they are. For instance, the comments about Mary are newer additions because they could only have been added after the writing, copying, and distribution of the gospel accounts. We know if the Talmud, for instance, an older writing, was tampered with if we see a comment like that added within it.
But a few comments, where Jesus is not countered exactly the same way or it is clear that the attempt to counter something in the gospels is added to a comment about Jesus, like making a sentence too long or sounding as if it was written by someone else, etc., it makes it clear that the first part might be about the real rabbi--but then that means we only have something like the name--Yeshua (Joshua or Jesus).
And, eh? So what. It makes sense. "That which is hateful to you, do not do it to another person. That is the entire Torah, and the rest is commentary. Go study." --Hillel (Shabbat 31a)
Hillel the Elder was a famous teacher who lived in Jerusalem during the time of King Herod. Hillel is a historical figure. We know that Hillel was real.
Scholars believe that whoever the Jesus of Nazareth rabbi was, he was probably a student of Hillel since the Golden Rule as found in the gospels, and many of the teachings of Jesus sound almost exactly like the teachings of Hillel.