Since I'm little more than a For Glory Hero, my usual secondary is Bowser, not for covering Ike's bad matchups, but for dealing with the infamous roll spammers that spend the whole fight running away when I'm feeling too lazy to make reads. His dash grab and Whirling Fortress quickly shut down any attempts at scrub tactics and Bowser Bomb ends their stock easily.
However, I used to play a lot of Pikachu in previous Smash games so I still pull out the little fella so I can work on my aggro game.
I usually cycle around Ganondorf, Dedede, Roy, Ryu, and Villager as well, to keep myself rounded on play styles. I've been working a bit on Donkey Kong, Kirby, Charizard, and Luigi, though I'm not really privy to their inner workings as much.