I'm not really one to speak in the department of a good secondary since I can play close to half of the characters in brawl and hold my own, but here it goes.
My "main alts" include Fox, Marth, Sheik, Ness, and Toon Link. I'll try to relate each to the playstyle or mechanics of Pit
Fox
Similarities:
Pivot steps like a beast, combos very well without risk of knocking your opponent far back, low lag after attacks, has an amazing vertical recovery, awesome taunts.
Differences:
Falls faster, attacks are released much faster, very easy to edge-hog gimp, obviously not as good at recovering, un-aimable projectile with no knockback, fewer mindgames, more pressuring, many kill-moves.
Why I Play: My friend/roommate mains Pikachu, so I have ready access to my worst matchup. Also, I love how fast his attacks are released, and how well he combos. Oh, and cool taunts.
Marth
Similarities:
Mid-high priority due to weapon, similar physics, combos well
Differences:
Most attacks are executed in one swift blow rather than many weaker attacks, comparatively horrible recovery (but that's not saying a lot). no projectile
Why I Play: Old main in Melee, know the character well, high KO potential at low %s, chains taunts to look awesome.
Sheik
Similarities:
Similar physics, combos well, projectile can be released at about the same speed to similar knockback, has numerous recovery options
Differences:
Combos much better on the ground, aerial attacks leave something to be desired, despite many options rather easy to gimp
Why I Play: Fun character to play, didn't have a ground-based comboer, ninjas are awesome.
Ness
Similarities:
almost none, I just play the psychic boy. :<
Differences:
they barely have anything in common. Ness is just awesome.
Why I Play:
Dair, Bair, Fair, Up-B, u-taunt.
Toon Link
Similarities:
Similar ability to recover due to Toon Physics, projectile-enhanced gameplay, great at comboing, rather threatening in the air, probably the most similar to Pit of this bunch.
Differences:
While the recovery abilities are stellar, they (like any other character) just aren't as good as Pit's. TLink is smaller, a little more nimble, and has a very fun (and often risky) d-air.
Why I Play:
<3 Windwaker, my favorite link encarnation, just so happens to be a lot of fun to play as well, d-taunt
I guess this list wouldn't be complete without including my reasons for playing Pit to begin with. I play Pit because of how versitle he is. You can switch from offensive to defensive in the blink of an eye. Even when being defensive, you still have attack options, be it scrambling or reflecting projectiles, shooting arrows as you recover, or edge-dancing with almost any attack. Pit wins chases, too, something that I've never been good at in Smash. I love his physics-warping ATs, everything from arrow-bouncing to wing refreshing. Glide toss can come out of nowhere. Arrows can come out of nowhere. All in all, if someone has never faced a decent Pit, they are in for a shock, and I want to make that shock as high voltage as possible. Most people have this impression that Pit is an arrow/angel-ring-spamming, flying noob character with an annoying voice. I want to prove these losers wrong.
Oh, and angels are awesome.
