I haven't listed every single one that I think has the advantage against Luigi in neutral, but hey, debate and discussion.
Needles + Mobility and overall comparable frame data means that Sheik isn't afraid of Luigi
Sheer speed, and the fact that Water Shuriken is such a versatile projectile (stuff Luigi with small ones, power through Fireballs with large ones)
While losing the neutral is a huge problem, he has the speed and power (and the armor) to avoid losing it in the first place
The one everyone mentions. While not as lopsided as originally thought, pellets, vastly superior air control, and just his overall projectile game are huge issues for Luigi
An arguably equally simplistic neutral game, what happens when dash grab meets dash grab? The fact that Meta Knight's launches him forward so much more (and that he can take to the air to avoid a lot of other stuff) is probably going to give him some kind of advantage.
If Mewtwo loses the neutral he gets bodied hard. However, between Confusion, Shadow Ball, and good overall movement speed, he has two options to take on Fireballs directly, and if Luigi takes to the air then up smash becomes a legitimate option.
Another character with a combination Charge Shot + Reflector
Charge Shot, long limbs, and Zair
Luma
Vastly superior mobility, combined with Bike to tear through things, a nasty command grab, and the threat of Waft to turn things around.