Halle Lidner, real name Halle Bullook, works for Near as part of his investigation team, along with Gevanni and Rester. She is mentioned to be a former CIA agent.
In the manga, Lidner knows Mello and leaks SPK information to Mello; Near is unaware of Lidner's SPK information leaking. When Mello questions her and asks her which side she is on(Near or his), She replies that she is on neither side, and merely helps both of them because they are after Kira. Later on, she calls Mello and tells him about Near's plan(Having Mikami write everyone's name down in the notebook), and he replies "I'm the only one who can do it.". This can be seen as the real push that drove Mello to kidnap Takada and eventually save the lives of both investigation teams.
Mikami views her real name, Halle Bullook, with his Shinigami eyes and proceeds to write her name as well as the other SPK members' names into his Death Note, but she remains alive due to the notebook being a fake.