Kreia is a clone of the former Jedi Master and Sith Lord, Darth Traya. Although she cannot officially accompany you outside of the Eagle, she can still be recruited and offer sage advice on certain matters. She is needed in order to convince T3-M4 to give you the coordinates to find Revan.
To have Kreia join, you must first deal with the situation on Etti VI (eliminating the Genoharadan's new boss and reinstating Kannos as the head). Once that is completed, you will be given the coordinates to Aperture Station. Travel there, and a cutscene will automatically take you straight to Kreia, and have her join through dialogue.