Rolo Lamperouge is introduced in the second season as an assassin
posing as Lelouch's brother, placed there to observe Lelouch following his
memory rewrite at the end of the first season. Rolo pilots the Vincent
Prototype, and possesses a Geass power in his right eye that allows him to
temporarily freeze the subjective experience of time for all persons within
a given range. He is manipulated by Lelouch into supporting the Black Knights and seems to become dependent on Lelouch over time, treasuring a locket given to him by Lelouch on his birthday and always referring to him as "brother."
As Lelouch digs more into his past, it is revealed that his real name is Rolo Haliburton and he is an assassin sent from the Geass Directorate. However, even he does not know its secret location. Due to his loyalty to his brother, he decides to betray the Directorate by joining the BK forces to attack VV's Siegfried. It is revealed that VV and Rolo knew each other, as VV references Rolo's Geass weakness. It is very likely that VV gave Rolo his Geass.