I think robusta is fairly upright, so not as V-shaped as other Fargesias. It's not as cold-hardy as listed though (depending on what part of the country you're in), as I know several people who have tried to grow it in z6 and had it top-kill. I hope you have better success with it than we did. It really is beautiful and I wish I could grow it here!

Although I don't have a specific companion plant recommendation, I'd say look for something large-leaved and relatively low-growing. Large leaf for contrast with the bamboo leaves, and lower-growing (a couple of feet) so it doesn't compete visually with the height of the bamboo. Maybe some of the larger hostas? I don't know if you have deer problems there or not, so hosta may not be the right choice, but they like the same conditions as the robusta.

Hydrangea might work for you.
The mop heads and lacecaps would give you good contrast.
The oakleaf Hydrangea would probably work as well,you may
have to do some pruning on it if starts to get tall though.