Well it really depends on what you're looking for. You might want to second a character who is much different than Falco. That's a good idea since you can have a lot of bases covered if you play someone who mains/seconds similar characters. But it's a also a little riskier, since you need to change mindsets completely and it's more difficult to learn 2 different characters at tournament level. If you're willing to do that though, Olimar isn't a bad choice. People don't see a ton of Olimars too and he handles differently. You could consider a heavy character such as Bowser, DK, or Snake. But they are easier to combo so its risky (plus people see a lot of Snake). Maybe Link? He's heavy and has a good projectile game with nice aerials. If you're afraid that he's considered lower tier then Wario is a good way to go. I'd probably pick him. He's different from Falco in a lot of ways, but has some fundamental similarities.
If you don't feel like taking the time to learn totally new characters, then Captin Falcon is a good choice. They both have similar recovery heights and are fast falling and quick paced characters, but aren't completely similar like Falco and Fox for example. But this is kind of a vague question you should play who you like really.
If you didn't feel like reading all of that I suggested Wario or Kkeeping Captin Falcon (I get a little carried away sometimes)