From the Super Mario Wiki
A Green Fireball is Luigi's trademark element in most spin-off games. It is usually functionally identical to Mario's Fireball. Other elements Luigi can use, however, includes electricity as seen in Mario & Luigi: Superstar Saga and Mario Strikers Charged and whirlwinds as seen in Mario Super Sluggers.
Super Smash Bros. series
Green Fireballs make their debut in the Super Smash Bros. series as Luigi's neutral special move. The move is called Fireball, the same name as Mario's move, but it floats rather than bounces and does slightly more damage. Kirby can use Green Fireballs if he copies Luigi's ability. Luigi retains this neutral special move in the rest of the Super Smash Bros. series, receiving no major changes.
In Super Smash Bros. for Nintendo 3DS / Wii U, the Green Fireballs can be altered into either Fast Fireball or Ice Ball, provided that they are unlocked first. Fast Fireball is affected by gravity, much like Mario's Fireball, although Luigi takes more time to fire it. In comparison, Ice Ball has a chance of freezing opponents and has longer range, but deals slightly less damage and travels slower.
Mario Kart: Double Dash!!
Green Fireballs appear in Mario Kart: Double Dash!!, as Luigi's special item and can be used only by him; not even Petey Piranha or King Boo can use it. It works similarly as Mario's Fireball item. When Luigi throws it either forward or backward, the green fireball splits into five and bounces around the course, which may ricochet off walls. The fireballs, when they hit any kart, including Luigi and his teammate's kart, cause the kart to spin out and the rear player is burned and momentarily stunned. They are one of the only two Special Items that can't be obtained by King Boo or Petey Piranha, the other being the Birdo Egg.
As well as having the special item, Luigi has a personal kart, the Green Fire, unlocked once players beat Mushroom Cup on 50cc.
Mario Superstar Baseball
A Green Fireball appeared in Mario Superstar Baseball as Luigi's Power Shot similar to Mario's Fireball move. When used while pitching, he hurls the baseball, encasing it in green flames, toward the batter at high speeds. If using it while batting, Luigi encases the baseball in green fire when he hits it and blasts it to the field. If a fielder catches it before it lands, he or she burns and drops the ball. If it lands, it lose its fire and cannot stun the fielders. In Challenge Mode, Luigi can use this special move only when the Green Fireball has been bought.
Mario Hoops 3-on-3
A Green Fireball is used in Mario Hoops 3-on-3, as Luigi's Special Shot, identical to Mario's Fire Shot. When the player taps out the letter L on the touch screen, Luigi starts charging the ball with green fire. If he is not hit while charging it, he launches the ball to the basket. It always scores if it is shot inside the 3-Point line. When shot outside the 3-Point line, a teammate must complete the move, and it is not guaranteed the shot scores.
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