The Talmud (Megillah 12b) states that Mordechai’s mother was from the tribe of Yehudah, while his father was from Benyamin. Although the tribal ancestry was paternal, members of the two tribes would later vie over his heritage to take credit for Mordechai’s greatness. The Alshich teaches that the verse is stressing that his mother was from the royal house of Yehudah. Rav Yehonasan Eibshutz says there is a mystical reason for this. According to the Talmud (Sanhedrin 99a), Haman’s female ancestor, Timna (see Bireishis 36:12 and Divrei HaYamim 1 1:36), also came from royal blood. Since, as Rav Elie Munk writes most cogently in Ascent to Harmony, “the division into masculine and feminine principles provides the pattern for all of creation” (80), the feminine aspect of Mordechai had to match the feminine aspect of Haman in order to defeat it. Therefore, Mordechai’s mother had to come from royalty to counter Haman.
R’ Yochanan’s opinion in the Talmud (Megillah 12b) is that Mordechai actually was from Benyamin, but was called a Yehudi because he fought against idol worship. According to Rav Yaakov Kaminetsky (in Emes L’Yaakov), the name “Yehuda” is especially apt for someone who stands against idolatry because the first three letters of the name (yud, hey, and vuv) are letters that spell the Name of H-Shem that represents His mastery over all, and is thus a rejection of pagan beliefs. The Chida adds that the gematria of Mordechai HaYehudi (40+200+4+20+10+5+10+5+6+4+10=314) is the same as Sha-d-ai (300+4+10=314), the name of H-Shem that connotes His ability to keep things from growing out of control, as in “He who said “dai!” (“enough!”)” while creating the borders of the world (Talmud, Chagigah 12a). Therefore, He controls everything, and can turn everything around, as He does through Mordechai and Esther in the Purim story.
The Sfas Emes gives three reasons for Mordechai’s being called “Yehudi,” all three relating the word in its guttural etymology to the word “echad,” (“one”). First, Mordechai was a “yachid,” (“a unique individual”) in that he saved an entire generation (see Midrash, Esther Rabbah 6:2). Second, he unified the Jewish people to counter Haman’s criticism that they were splintered in disunion (Esther 3:8). Finally, Mordechai sacrificed everything for H-Shem who is One, Echad (Devarim 6:4).