I actually think fox completes falco best. I'm maining both spacies now personnaly.
Fox is the best at falcos weaker MUs
and probably better with
. And he's very fun to play too, who doesn't like to go fast ? The biggest drawback is that switching between these two is very disorienting every time, mainly beccause of fox's shorter jumpsquat.
If you wanna choose between Marth and Shiek, I guess go with Marth for the MU coverage and usefulness of the things you'll learn (movement stage control spacing etc.). + he's infinitely more stylish than shiek.
Shiek on the other hand is WAY easier to git gud with (something you might want to consider, especially for a secondary). From my experience she's the easiest character to learn and to play, but also no fun at all.
Finally you can choose the option not to train a secondary. Sticking whit the bird sure gets horribly frustrating at times (like when my best friend picked up jiggly for the first time ever and 3-stocked me 10 times in a row. ugh.) but there's nothing you can't overcome with him, and ultimately it's very rewarding.