Luigi (SSB4)/Neutral aerial
Luigi's neutral aerial is a standard sex kick, with the same animation as Mario's neutral aerial. Like said move, it has very fast startup (coming out on frame 3), tied for the second fastest neutral aerial only behind Little Mac's, has long-lasting hitboxes with a clean hit and a late hit, is interruptible on frame 46, and autocancels during the first two startup frames as well as four frames after the hitboxes cease.
However, and despite seeming like a cloned move at first, Luigi's neutral aerial has several mechanical differences from Mario's. Luigi's clean hitbox on his stretched leg is 0.5 units larger than Mario's, giving it slightly more range; conversely, the late hitbox on his bent leg is 0.5 units smaller. Its late hit lasts 2 more frames, giving the move a longer duration (29 frames compared to 27), but it also autocancels 2 frames later as a result (on frame 36 rather than 34), and has higher landing lag (14 frames rather than 10). Furthermore, it deals more damage overall than Mario's, with 12% on the clean hit and 6% on the late hit, compared to Mario's 8% and 5% respectively, and has different knockback values. Lastly, and as the most important difference by far between both moves, Luigi's neutral aerial launches opponents almost entirely vertically (at 90° with the clean hit and 80° with the late hit), rather than at the Sakurai angle, sacrificing edgeguarding ability for combo potential and distinguishing it apart from all other prominent sex kick moves in this aspect.
Luigi's neutral aerial is not only a very unique move in its own right, but also one of the most potent sex kicks in the game overall. Due to its very fast startup, large clean hitboxes that cover Luigi's entire lower body with some disjoint, and disproportionally high damage for its speed, this move holds infamous recognition as an excellent combo breaker, even more so than other attacks of its speed; this is further compounded by Luigi's combination of middle weight and floaty vertical movement (with both his falling speed and gravity being below average), which cause him to be launched farther away by traditional combo moves while taking comparatively lower hitstun, thus reducing the opponent's hit advantage on him.
On top of the aforementioned strengths, Luigi's neutral aerial is also his second strongest aerial, beaten only by his sweetspotted back aerial; at ground height, the clean hit KOs middleweights of average falling speed and gravity at around 160% in Final Destination without rage, and 133% with max rage, while floatier and/or lighter characters are KOed at around 140% and 115% respectively under the same conditions. Due to its perfectly vertical angle, directional influence has an infinitesimal effect on the move, allowing opponents to survive only 2%-3% longer.
With this move, Luigi can not only discourage practically any non-guaranteed followup, but even start combos of his own if he lands quickly enough after hitting the opponent, due to its vertical angle and moderate landing lag. As such, it serves both defensive and offensive purposes, getting Luigi out of a disadvantageous state while simultaneously setting up powerful punishes on opponents that either whiff their attacks against Luigi or use them too late. In the latter regard, it is particularly versatile due to the clean and late hits possessing different damage values and knockback scaling — and therefore different resulting knockback —, allowing Luigi to maximize combo damage at lower percents using the clean hit, then extend the move's combo potential at higher percents using the late hit.
Both the clean hit and late hit of neutral aerial can combo into virtually any of Luigi's standard ground attacks at low to mid percents where its knockback is low enough to not cause tumbling. His most effective followups are an up tilt at percents where it becomes safe on hit (around 30% on middleweights), which can be chained up to 3 times at worst for roughly 29.49% damage and be followed up further with other aerial attacks afterward; a grab, which can be comboed into several aerials out of a down throw for similarly high damage; and a neutral attack, which is Luigi's fastest ground attack for situations where the neutral aerial's hit advantage falls short for any other option (including the former two) to be a true combo.
At mid to high percents where it starts causing tumbling, the move can combo into virtually any of Luigi's aerial attacks, with the most effective followups being a chain of forward aerials; two up aerials; or a late up aerial into a back aerial, all of which deal roughly 21% plus the neutral aerial's damage. Comboing into a clean down aerial can also meteor smash the opponent back to the ground and allow Luigi to potentially start another combo by successfully reading their reaction. Lastly, other more situational followups are Super Jump Punch and Luigi Cyclone, both of which are harder to get consistently, but can KO off the top blast line with the assistance of platforms and if Luigi has enough rage.
Neutral aerial's use for KOing opponents is situational, due to Luigi having several other moves that can reliably KO earlier such as his smash attacks, back aerial, and Luigi Cyclone (either offstage to gimp opponents, or onstage and close to the top blast line), as well as neutral aerial being prone to stale-move negation due to its utility as a damage racker and combo breaker. However, it can still come in handy at further discouraging opponents from comboing Luigi high above the ground if they are at high percents, especially if he has some rage, as this can result in the neutral aerial cleanly KOing them with barely any chance to survive longer via optimal DI.
While having a plethora of benefits for several situations, Luigi's neutral aerial also comes with some notable weaknesses. Due to Luigi's very slow air speed, floaty physics, and the move's long duration and rather short range, approaching with it is a poor, easily punishable option, as using it immediately out of a jump or high in the air renders Luigi vulnerable for several frames with little chance to back away, and its landing lag makes it unsafe on shield unless he lands nearly immediately after hitting with the move, which is difficult to achieve consistently because of his floatiness. It is also unsafe on hit at low percents if Luigi doesn't land with it; from 0% to 20%, opponents of all weight classes have from 26 to 16 advantage frames over him if he strikes them with the clean hit and doesn't land at any point, more than enough for them to retaliate with an aerial of their own. As a consequence, while generally perceived as a very low risk move that can get Luigi out of any situation, there are actually several factors players need to take into account to use it effectively, as it can otherwise have the opposite effect, leaving them even more vulnerable to punishment.
Overall, like in its two previous appearances, Luigi's neutral aerial is one of his best moves, as well as one of the best neutral aerials in the game, but has weaknesses that can be easily exploited by a smart opponent, and being aware of them at all times is required to properly take advantage of its benefits.