From the Super Mario Wiki
Booster, also known as Booster the Seventh, is a character from Super Mario RPG: Legend of the Seven Stars who lives in Booster Tower with three Snifits. He is of the seventh and current generation of Boosters. Despite his wealth, Booster would rather have fun and spend his days playing with his Snifits and others he invites to his tower.
Super Mario RPG: Legend of the Seven Stars
When Exor crashed into Bowser's Keep, Mario, Bowser and Princess Toadstool went flying in opposite directions. While Mario landed safely at his Pipe house, Toadstool, unfortunately, landed at Booster's Tower. Of course, Booster's reaction to this strange event was even stranger; Booster decided he must marry this woman who fell from the sky. Booster locked Toadstool on the balcony of his tower, waiting until the time was right to marry her, despite her constant cries of "MARIO, HELP ME!"
When Mario arrived at the tower, Booster thought he just wanted to have fun. Oblivious to who Mario was, Booster determined the mustachioed hero before him was none other than the "Mario" Toadstool had been screaming about. Booster attempted to eliminate Mario with some Bob-ombs, but Mario escaped the deadly blasts.
After playing a game of hide-and-seek with Mario (and recovering the Mario Doll), Booster flees the tower, taking Peach with him. Mario gives chase, but first has to battle Booster's clown minions, Grate Guy and Knife Guy. After dispatching the duo, Mario chases Booster up Booster Hill to Marrymore, the marriage resort where Booster plans to marry Toadstool. Mario crashes the wedding.
In the end, Booster ends up helping Mario and his friends by eating a monstrous wedding cake that prevented the team from leaving the wedding chapel. After the Marrymore incident, Booster and his Snifits returned to Booster Tower to resume their games and fun.
If Mario returns to the balcony on Booster's Tower after the events of Marrymore, Booster can be seen on his tower balcony with his Snifits. He is staring at the sky, thinking another potential wife will eventually fall down to him. Later, if Mario returns after the events of Nimbus Land, a second woman fell from the sky and landed at Booster's Tower, Valentina, with her bumbling assistant Dodo. Valentina at first rejects Booster, but Booster whispers into her ear something unknown into her ear that has the profound effect of winning her over.
The two villains soon fall in love and eventually go to Marrymore to get married, with Dodo presiding over the wedding, as seen in ending. However, Booster seems to get cold feet with Valentina constantly getting closer to him, and he flees the wedding.
While Mario is in Marrymore, he actually may get a kiss from the bearded Booster, depending on how fast the plumber retrieves Toadstool's accessories. This happens if Mario takes a long time recovering the items. Other variations of the cutscene include Toadstool kissing Mario, Bowser kissing Mario and Booster kissing Bowser.
Booster appears with everyone else at the parade in the game's ending. He is riding a train following Valentina and Dodo. At a point, the Snifits run past with bags and Booster jumps out and follows them. The four are later seen fleeing from Croco and a Crook with the pair's loot. For some reason, Booster is in the Koopa Clown Car and Bowser is chasing him.
Booster is a bit crazy and relies on his Snifits to do all his work and explain things to him. The only time Booster wants to fight Mario is when his Snifits find him behind the curtains (when unsuccessfully hiding from them). Though even then Booster believes that it is the Mario Doll that he is fighting and not the real Mario.
Booster tends to be either ignorant or even naïve at the time, even not recognizing when Peach is crying.
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