Radja Nainggolan is indeed playing for Cagliari, but he's had some attention from other Italian sides, most notably from Napoli.
All I can say about him is that he is decent. He is a defensive midfielder and one of the combative kind. He is booked pretty often.
But since Belgium has guys like Fellaini, Dembele, Simons, Witsel, Odjidja and Defour competing for a box-to-box/defensive midfield spot, Nainggolan has played only on two occasions for Belgium.
I'd consider him a bit similar to Defour: equally combative, also a short guy, but he tackles a bit more and passes a bit less well.