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 is one of Bowser's lieutenants before the events of Super Mario RPG: Legend of the Seven Stars. However, when Smithy invades and takes over Bowser's Keep, Jagger is forced to desert the Koopa Troop. Realizing he is not strong enough in his present state to fight the forces of the Smithy Gang, he flees to Monstro Town, a village of reformed monsters.
Once there, Jagger meets the martial arts master Jinx. Jinx, seeing potential in the young 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.
When Mario and Bowser arrive in Monstro Town, Jagger is shocked to see his former King. He apologizes to Bowser for running away and promises to immediately join his King to reclaim their castle. Bowser, in a rare display of humanity, tells Jagger that his services are not 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 instead of rejoining his Koopa forces.
As a test, Jagger challenges Mario to a fight. Although Jagger is a skilled martial artist, he ultimately loses to him.
After Mario defeats Jagger's master, Jinx, Mario becomes the new Sensei. Thus, Jagger and Jinx begin to train in Mario's style of martial arts. Their first training assignment is to Jump one thousand times.
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