Booster is a character featured in Super Mario RPG: Legend of the Seven Stars. He's a large man with an odd beard, a constant, almost unchanging facial expression and a Viking hat. He resides in Booster Tower and is quite childish. His castle is one of the largest structures in the game, and thus Booster travels it via a toy train that he's created to go across the entire building.
According to Booster, Princess Toadstool fell from the sky and onto his tower. Ever since he's been holding her captive, awaiting a day that the two can get married over at Marrymore. Mario and Bowser both arrive at Booster Tower and subsequently hear Toadstool yelling for help from the top of the tower. The two former enemies decide to unite to save her, both with their own reasons for doing so. Bowser barges through the door and the party manages to get into Booster Tower.
After traversing the area for awhile and defeating the opponents, Booster comes in on his toy train, welcoming the group to his "famous tower of amusement". He says that he would normally welcome them, though that a girl (Toadstool) fell out of the sky and thus he has to keep her happy. Nevertheless he says for them to enjoy their stay and thus heads off.
On the second meeting, Booster is paranoid and rightfully so. He asks Mario why his "bride-to-be" is uttering "MARIOHELPMEMARIOHELPME", and wants to know if it's either a sign of happiness or if in fact this person he's talking to is in fact the Mario she's yelling for, and thus sends a barrage of bombs down. Mario can easily dodge them by either exiting the room or hiding behind the wall.
Finally the group manages to get to the top where the princess is being held, though the door to her has been locked and only Booster knows the secret to opening it. Peach tells Mario to hide behind something when they hear Booster coming, and thus he hides behind a set of curtains.
Booster enters the room and asks his Snifit buddies if they think he should marry Toadstool. They offer him a party, though he's not entirely sure what a party is, and after hearing about one concludes that drinking punch and eating cake sounds complicated, and thus decides to rehearse it using a Toadstool doll. He says that they should take into consideration that Mario might arrive and ruin everything, and thus requires that the Snifits find his Mario doll in order to act out how that may occur. They can't find it, so he orders them to look behind the curtains.
Depending on how well the character avoids the Snifits, he'll either have to battle Booster or won't have to battle him. If he gets caught behind the curtain three times, then he'll have to. Otherwise he'll move on. After the three Snifits can't manage to find the doll by looking behind the curtains, Booster assumes that opening and closing curtains is fun and opens one himself. Mario is behind the one he opens, though right then he looks above and sees the Mario doll sitting there, and doesn't even pay attention to him. He starts to bawl because the doll is too high, and Mario jumps beneath it so that it'll fall of next to Booster, at which he thanks him, conclusively assuming that he doesn't realize that he was thanking Mario.
After this, Booster says that there's no time for rehearsal and sets off with Peach. Mario and group go after them, though are forced to battle the towers' guards Knife Guy and Grate Guy. After defeating them they head to Booster Hill, where they chase after Booster up to Marrymore.
At Marrymore, Booster kicks Raz and Raini out of the wedding hall during their ceremony so that he and Toadstool can get married. His comrades barricade the doors, though they manage to open them up by cooperatively knocking into them at the same time. Mario and Bowser crash into the Snifits who bump into Peach, thus causing her to lose all of her gear. She starts crying because of this, and for an odd reason Booster things she's leaking and tastes the tears, saying that it tastes salty. Booster orders her Snifits to find her gear so that the tears don't get on the wedding cake, and thus they start searching. Mario ends up finding everything and gives them to Booster.
Booster quickly notices that this was Mario, and asks if he's come to crash his wedding. Peach kisses Mario for saving her, and Boosters says that he wants one too. He goes in for one and, depending on how fast Mario obtained his items, won't get a kiss at all or, worse yet, end up accidentally kissing Mario.
Meanwhile, the chefs are extremely angry over the fact that no one can eat their cake they made for the wedding, and thus enter battle with the group. The cake eventually comes to life, and the chefs escape. Booster eventually comes and gobbles the cake up, says that it's a wrap thinking that he's married and leaves without the princess. This is the last you see of Booster until the end, when he's seen going after another female - Valentina.