Commander Tomalak and Commander Sela, it seems like these two characters were set up to be Picard's Romulan counterpart and nemesis at different points of the show, but nothing really came of them. It seems like a bit of shame, but both of them had a pretty good build up, made some memorable appearances, but didn't appear enough to have any sort interesting rivalry with Picard, like say Sisko and Dukat had.
It seems like a bit of a shame and waste, I think one of the reasons why the Romulans come off as underdeveloped compared to other civilizations is there was never major Romulan reoccurring character that made a real impact, while every other civilization of importance had one: Vulcans, Klingons, Cardassians and the Ferengi, heck even 7 0f 9 as an ex Borg drone gave us some insight into the Borg that Romulans were lacking. If either Sela or Tomalak had been more developed and better used, that would given us more more insight into the Romulans beyond being evil and sneaky.
It seems like a bit of a shame and waste, I think one of the reasons why the Romulans come off as underdeveloped compared to other civilizations is there was never major Romulan reoccurring character that made a real impact, while every other civilization of importance had one: Vulcans, Klingons, Cardassians and the Ferengi, heck even 7 0f 9 as an ex Borg drone gave us some insight into the Borg that Romulans were lacking. If either Sela or Tomalak had been more developed and better used, that would given us more more insight into the Romulans beyond being evil and sneaky.