Rabi (Rabbi Yehuda haNasi - Tannaim - Sixth Generation, c.170 - c.200 CE) succeeded his father, R. Shimon ben Gamliel as Nasi and was a key figure in Jewish history. A pious and brilliant scholar, he worked to further consolidate the authority of his office. His great wealth and his prestige among the Jews also impressed the Romans, resulting in many discussions with them. His greatest achievement, however, was his organization and compilation of Jewish Law in the Mishnah.
Biography: Rabbi Judah HaNasi was born to Rabban Shimon ben Gamliel II in 135 CE (and died in 200 CE), which was the same year that the Romans suppressed the Bar-Kochba Revolt and only 65 years after the Romans destroyed the Second Temple. The national trauma was still fresh, and the Sanhedrin was hunted by the Romans and thus not providing any unifying leadership. Without a central address to settle questions, the Oral Tradition was in danger of being lost. When Rabbi Judah became the first Nasi (Patriarch - same Hebrew title, but different function than the President of the Sanhedrin) in 161 CE (some say from 170-200 CE), he resolved to solve this by collecting all of the sayings of the rabbis and organizing them into 6 categories ("orders", or "sedarim") and 63 sub-categories ("tractates", or "masechtot"). Rabbi Akiba and his student Rabbi Meir had already had a go at this, so some of the work was done in previous generations. Not all of the sayings made it into the Mishnah; those were collected and called "baraitas", and they frequently appear in the Talmud (the "Tosefta" is a collection of baraitas). It's not clear that Rabbi Judah HaNasi wanted to "fix" the law; he may have intended to only write down what everybody thought so that future generations could decide what to do. For his efforts, Rabbi Judah HaNasi is sometimes just called "Rabbi".
- David Schwartz, Pirkei Avot Ch. 2 - With Bios