Okay.
R.O.B. is a defensive heavyweight projectile / trap-based character. He needs to zone the opponent out and then setup for kill confirms. He's a great spacer and zoner with a solid advantage, but his disadvantage isn't very good and he doesn't have much responses to rushdown pressure - ergo, he can struggle with the top-tiers.
What you want, then, is a character with good CQC options that can respond to pressure. Pit's good in that respect, he's got a good jab, exceptional dash and pivot grabs, a cross-up option in side-b and he can space well with tilts and airs. The big condition with Pit is that you need to know how to play him to get the best results, since he relies on good fundamentals and footsies to get the job done. He's easy to pick up as a secondary, that's fair to say - I just don't know if he really checks R.O.B.'s weaknesses, rather than just covering them up.
Have you considered Mario? He's got a good projectile to set up for combos and he can easily reset to neutral with n-air, something R.O.B. can't really do. He can space and force the approach with fireball, but in contrast to R.O.B., he wants to be on top of the opponent and keep on the pressure. He's quick enough to not have to worry about getting pressured himself, but because his zoning game isn't as good as R.O.B.'s, his combo setups and kill confirms have to come from his own reflexes and footsies. His edgeguarding is pretty top-notch, so that's another potential weakness of R.O.B.'s that he can check.
How's that?