I've noticed that a good Olimar both is easy to adapt to as a ROB main, and solves a lot of problems ROB has. From my past few plays on Smashladder, I've been able to deny a tourney-tier Pikachu who beat my ROB because Alph was so good at spacing and his airs compared to ROBs are godly. I'd suggest trying out Olimar and seeing how he feels, though he might be harder to adapt to and understand what Pikmin are good in what situations.
Basically, like ROB, Olimar/Alph is good at everything, be it heavy offense or tricky defense. A heavy focus is put on his projectiles, just like ROB's Gyro. However, he's smaller, thinner, can move a bit faster, and his airs are much better than ROBs. I'd recommend him, especially against more combo-centric characters like Pikachu, who give ROB a real run for his money.
I dunno, maybe I'm just much better at Alph then I am at ROB, but coming off an 8-win streak and only losing as Rob (who I always play first round as) once, I'd have to say that the two play off eachother very well.