Nyxeris is an asari on Illium who is an assistant to Liara T'Soni. Secretly, Nyxeris is an agent of the Shadow Broker, known as the Observer. During Shepard's mission for Liara to investigate the Observer, her true identity can be revealed by Commander Shepard or she may stay undercover due to lack of evidence. If Shepard tells Liara that the Observer is a female she will call Nyxeris into her office. The next time Shepard goes to visit Liara, she mentions that she killed Nyxeris, saying that "her barriers needed practice. Practice I'm afraid she won't be getting."
Nyxeris' name is a combination of the names of Nyx and Eris, the Greek goddesses of night and discord, respectively.
With the release of the Lair of the Shadow Broker, it is possible to tell Liara the relevant intel, causing her to return home, before having completed the The Observer mission. In this case, near the end of Shepard and Liara's conversation, Nyxeris walks off screen (never to be seen again), and the journal updates to report the mission as complete.