Star Trek: The Next Generation

Commander Sela was a Romulan-Human hybrid; her mother was Lieutenant Natasha Yar of the USS Enterprise-D from an alternate timeline, while her father was a Romulan general.

In 2344, a temporal rift transported the USS Enterprise-C to the year 2366. The starship had been in the process of answering a distress call from Narendra III, a Klingon outpost that was under attack by the Romulans. With the Enterprise-C absent from its original timeline, hostilities continued between the Federation and the Klingon Empire, resulting in open war from the early to late 24th century. In this timeline, Natasha Yar had not been killed at Vagra II and continued to serve in Starfleet. When the Enterprise-C returned to its correct position in the timeline, Yar traveled with them, knowing that to remain in 2366 would result in her dying a senseless death.

(TNG: "Yesterday's Enterprise")

When the Enterprise-C returned to its original timeline, it was destroyed. Yar and several other crew members survived and were taken prisoner by the Romulans. Yar agreed to become the consort of a Romulan in exchange for the other crew members being spared from execution. One year later, the couple bore a child, Sela. In 2349, Yar tried to escape from Romulus and attempted to take Sela with her. Unfortunately, Sela called out when she recognized she was being taken from her father. Consequently, Yar was executed and Sela was henceforth raised completely Romulan.

(TNG: "Redemption II")

Over the next fifteen years, Sela rose to the position of commander in the Romulan military. By 2367, she was a high-ranking officer and was involved in the kidnapping of Geordi La Forge for the plot to assassinate Klingon governor Vagh of Krios. She also organized the Romulan support of the Duras sisters during the Klingon Civil War, when her existence and the alternate timeline created by the Enterprise-C was revealed to the Federation.

(TNG: "The Mind's Eye", "Redemption", "Redemption II")

Sela was encountered again later in 2368, when she masterminded the Romulan plan to invade Vulcan in collaboration with Proconsul Neral and Senator Pardek. Once she had lured Ambassador Spock to Romulus, Sela created a holographic replica of Spock called Spock One to provide a cover for the Romulan invasion force. Unfortunately, she under-estimated Spock's ability to access the Romulan computers, allowing her prisoners to create a holographic diversion and escape. This led to the failure of her venture.

(TNG: "Unification I", "Unification II")