He is tied for 3rd on the tier list with his brother. Luigi has excellent horizontal movement, high jumps, and good KO power. He has easily the best projectile in the game, the Fireball, due to its very fast speed and firing rate. The Luigi Cyclone is a good turtling move, as it comes out fast, surrounds Luigi on both sides, has a long active period, and has instant-KO properties.
Like Mario, Luigi does not have any real weaknesses, other than his lack of a momentum cancelling attack. This lack of downsides makes his strengths appear even more pronounced, as he has strong matchups across the board, resulting in his high placement.
Luigi shoots a fast-moving Fireball, which is unaffected by gravity. This move can be held down indefinitely, making Luigi skid forward a little each time he shoots a Fireball. It should also be noted that performing this move again while another fireball from either of the Mario Bros. is still onscreen cancels the previously existing one out.
Luigi delivers two punches rapidly, the second only for a single frame; pressing twice in succession makes Luigi kick. Holding down the attack button produces the same effect. It is good for racking damage and KO'ing, due to its decent knockback.
Luigi uses Green Missile, launching himself sideways like a rocket and travelling a fixed distance. Strangely, if the attack gets continued by holding down the button, from the third time it repeats onwards, there will be a small delay which briefly returns Luigi to his idle stance before he can resume the attack.
Luigi uses Luigi Cyclone, spinning around in midair with his arms out. Like his down attack, it can hit multiple times. This move only has small horizontal knockback, but it is very useful for dealing damage.
The Fireball itself behaves very differently from Mario's.
It comes out 3 frames slower (5 → 8).
It travels in a straight line, as opposed to a sine wave.
It moves about half as fast.
This allows it to deal more damage overall due to overlapping the opponent for more frames.
It can also combo into itself at low percents, making it good for damage racking and early edgeguards.
It lasts 15 frames longer (23 → 38).
It travels a much shorter distance.
It does not grow bigger towards the end of its range.
The move transitions into jump attack 1 frame later when used in the air (16 → 17).
Luigi's firing animation does not auto-transition into his falling animation.
This makes it more difficult to spam, as the move will frame-skip into a useless falling animation if the attack button is held down. Luigi must turn-cancel during the time between firing the projectile and his attack refreshing in order to fire at the optimal rate.
Luigi's hurtboxes are bigger overall, especially during the initial frames.
It transitions into jump attack 1 frame later (16 → 17).
Luigi does not auto-transition into his falling animation.
This makes it more difficult to spam, as the move will frame-skip into a useless hurt animation if the attack button is held down. Luigi must turn-cancel before his attack refreshes in order to immediately follow up with a second down aerial.
The attack's hitbox is significantly wider and slightly taller, giving Luigi better coverage and range on the sides.
Luigi's hurtboxes are less consistent, being much bigger than Mario's on average.
This also means that Luigi does not have a disjoint above him.
Luigi has a short "wind up" animation at the beginning, giving him a strong disjoint on both sides during the initial frames.
Luigi does not "hop" at the end of the attack, giving his hurtboxes less vertical movement.
The attack's hitbox is bigger, giving Luigi better coverage.
It has slightly more range above Luigi and less range in front of him.
Luigi's hurtbox is noticeably wider.
Luigi's standing hurtbox is slightly wider and notably taller.
His crouching hurtbox is slightly wider.
His running hurtbox is a bit taller.
His falling hurtbox is somewhat off-center.
His getting hit hurtbox is slightly bigger.
As it was with the case of Mario, Luigi's original sprite sheet by Xander actually has elements that were omitted from SSF due to limitations, such as Luigi hitting with his rear or the coins from Super Jump Punch.