I actually mained ROB for a while and I came up with the conclusion that Snake and G&W are his best secondaries (depending on your preference)
I picked Snake because, between ROB, Snake, and G&W (which I played all three, but I wanted to decide a second one on which to focus more), I was most comfortable fighting Meta Knight as Snake.
For me, though, I realized I was just better with Snake than ROB.
Anyway, the only problems in this is that this leaves the ROB fight to a ditto, since Snake vs ROB is not very good. G&W would cover the ROB fight better. But same with G&W, you would have to ditto it.
Then with ROB / Snake you have DDD as a problem, neither is a good character to use against him.
So my all conclusive conclusion that concluded my thougts was, to have yourself covered as ROB, learn:
Snake / G&W
or
Meta Knight.
Thank you, that is all.