The Romulan senator who allies with Spock in “Unification” is Pardek.
Pardek has maintained a secret dialogue with Spock for several decades, focused on improving relations between the Romulan and Vulcan peoples. Sarek identifies him as the key Romulan contact when Picard investigates Spock’s activities on Romulus.
Pardek helps Spock and the disguised Picard and Data navigate Romulan politics. He explains that Spock has been invited to meet a reform-minded Romulan leader, although the apparent peace initiative is ultimately connected to Commander Sela’s invasion plot. Pardek’s cooperation makes him an important link between Spock’s reunification movement and Romulan political circles.
A common mix-up is Sela, the Romulan commander who opposes Spock, or D’Tan, a young Romulan associated with the underground movement. Tomalak is another Romulan commander, but he is not Spock’s senator ally in this story.