Lagan Serra was a female Bajoran in the 24th century. During the Cardassian Occupation of Bajor, she was a Resistance fighter. Following Bajor's admittance to the Federation in 2376, she became a Federation Ambassador at large. She was noted for her regal bearing, which belied her freedom fighter past.
In 2379, Lagan served as the Federation's representative on Tezwa. When Prime Minister Kinchawn learned Presidential Chief of Staff Koll Azernal was plotting to oust him, Kinchawn expelled Ambassador Lagan, and launched an offensive against both the Federation and the Klingon Empire. Kinchawn was later overthrown and exiled, and Lagan was able to return to Tezwa, where she negotiated with Kinchawn's former deputy, Bilok, for Federation aid and rebuilding assistance. (TNG novel: A Time to Kill)
While overseeing these efforts, Lagan unwittingly authorized the complete dismantling of the planetary communications network. When this order was discovered by Geordi La Forge, it led to the discovery of Federation President Min Zife's role in illegally arming Kinchawn during the Dominion War. Lagan was present for the arrest of Kinchawn, and then to bring these findings to a select group of Starfleet admirals.
By 2386, Lagan was serving as the Federation Ambassador to the Cardassian Union. Castellan Elim Garak would see it as the latest in a long line of insults that the Federation Diplomatic Service had inflicted upon him. (ST - Section 31 novel: Control, DS9 novel: Enigma Tales)
In late 2386, Lagan would be telepathically coerced by Section 31 director, Azura, into authorising a fake Federation Security Agency extradition request that Castellan Garak release Julian Bashir and Sarina Douglas into her hands. (ST - Section 31 novel: Control)