Pit is sort of like a Sheik...but with swords, wings (4 jumps), and needles that can hit anywhere. And he has Cap. Falcon-like D-air strings. And a Jigglypuff-like wall of pain. And a victory cheer that'll instantly make your friends 5x saltier than normal.
He relies on grabs similarly to Sheik, and can combo fairly well and easily like her too. D-throw leads to Up-B (kill% / floaty), U-air (high% / average), U-smash (low% / heavy), or tech chases (spacies). He has good/even matchups against most if not all of the cast. Once you train aiming your arrows, you should be able to hit anything from anywhere, which is pretty useful for gimps / tacking on extra % / approaches. Pit's Side-B also makes for a great approach option. F-air kills like Sheik's with similar range/etc., and his B-air is a weaker version with a lingering hotbox. N-air is good for combos, D-air has a Marth-like spike hotbox, sour-spotted U-airs combo, and sweet-spotted U-airs can kill. D-tilt and U-tilt work similarly to Sheik's, and F-tilt is a second F-air.
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However, you should still consider that there are few significant changes that would detract for your gameplay, if you're going from SSBM Sheik to PM Sheik. Since the reverse isn't true with the addition of SSBB mechanics in PM, which would likely mess you up going back to SSBM, the following info would only be useful in case of like...you practice SSBM Sheik primarily, you're playing PM, and felt like Sheik would have a better MU than whoever you pick as your PM main. Sorta like a last resort, if u feel your SSBM Sheik play is your best.
If you're trying to go straight from Melee into PM and play like a Melee Sheik, the main difference is that she loses her chaingrab. From a D-throw, you can either tech chase (not nearly as easily as in SSBM imo, usually only with spacie-weights) or F-tilt/etc. combo. Unlike in Melee, comboing into death (or at least lots of damage) isn't as easy, so you may find yourself resetting to neutral way more often. However, so long as you feel you have a good neutral game, you'll likely manage the SSBM --> PM transition.
PM --> SSBM can really hurt your play, in that you may not have enough chain grab and/or tech chase practice from PM Sheik to play SSBM Sheik. You could also fall into the habit of using B-throw into F-air (works pretty consistently unlike in Melee) and RAR (Reverse Air Rush) B-airs to space if you practice PM Sheik primarily. RAR airs are a Brawl mechanic that involves you pivoting during a dash and immediately jumping, allowing you to keep your forwards momentum while turned around to face backwards (which opens up the great spacing option for Sheik of dashing up to opponents and quickly SH RAR B-airing).