From the Super Mario Wiki
“Bowser! I haven't seen you for so long. I've been studying here at the dojo under the Sensei 'Jinx' because ... To tell you the truth, I... I know I couldn't beat Smithy. That's why I'm here now. Someday I'm going to return and make you proud!”
Jagger was part of the Koopa Troop before the events of Super Mario RPG: Legend of the Seven Stars. However, when the Smithy Gang invades and takes over Bowser's Keep, Jagger is forced to desert Bowser. Realizing he is not strong enough in his present state to fight the forces of Smithy, he flees to Monstro Town, a village of reformed monsters. Once there, Jagger meets the martial arts master Jinx. Jinx, seeing potential in the youthful and inexperienced Terrapin, allows Jagger to become his student. Jagger trains day and night, hoping to become strong enough to eventually challenge the forces of Smithy and reclaim Bowser's Keep.
If Mario and Bowser visit the dojo in town, Jagger will be shocked to see his former king. He apologizes to Bowser for running away and promises to someday join him in reclaiming his keep. Bowser, in a rare display of humanity, tells Jagger that he is not obligated to do so and that his services will not be needed, as Bowser has acquired new "Troopas" (namely Mario, Mallow, Geno, and Toadstool) to vanquish Smithy. Bowser, in fact, encourages Jagger to continue his training and self-realization rather than rejoining the Koopas. As a test, Jagger challenges Mario and his allies to a friendly fight.
Although Jagger has attained a rare level of skill, he ultimately loses and his master, Jinx, will reveal himself. If Jinx is defeated three times, Mario will be given an honor held only by the highest-ranking fighter, the Jinx Belt, and will be told he is the new Sensei. Thus, the dojo becomes "Mario Style Dojo", and Jagger and Jinx study Mario's fighting technique. Jinx proclaims their first training assignment is to jump one thousand times, which Mario does not have a say in. They continue to do this endlessly for the rest of the game.
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