This half-human blond Romulan was a surprise player on the Romulan stage for a brief span in 2367-68, leading the effort to back the Duras faction during the Klingon civil war and break up the Klingon-UFP alliance and later coordinating the invasion of Vulcan under the guise of reunification.
Upon meeting Sela, Picard and his crew were fazed when Guinan seems to back up Sela's claim that she was the daughter of an alternate-universe Tasha Yar, sent back in time to this universe, and a Romulan general who captured her ship in 2344. Sela was born a year later, but at age 4 caused her own mother's death during an escape attempt and Yar was killed.
That incident revealed the ruthlessness that probably helped make Sela a commander at 23, along with the ultra-Romulan outlook and passion that she adopted as a result. Only weeks later, her own arrogance and ego in maintaining lax guards of prisoners foiled the Vulcan invasion, and she was not encountered again by the UFP.