Years ago when Marianne was still a commoner met a guy who she loved deeply. He was of the Lamperouge family. They were deeply in love was about to get married. Marianne was a knight to the Britannia and she gained attention of the current emperor, Charles due to her fantastic piloting skills hence named her with the nickname of "Marianne the Flash". Charles was also in love with Marianne who did not love him. In order to get her, Charles threatened her with Lamperouge's life. Left no no choice, Marianne decided to marry Charles. On their last day together, Marianne and Lamperouge made out in bed. Charles made Marianne one of his wives after that. Soon after, Marianne was pregnant. As the time coincides, Charles did not suspect anything. Marianne's son was finally born. He resembles his mother. He was named as the eleventh prince of Britannia, Lelouch vi Britannia. A few years later, Marianne gave birth to Nunnally who is of the royal blood.
A few years later, Marianne and Lelouch met Lamperouge who had remarried and had a 6 years old son named Rollo. That's when Rollo had a glimpse of the prince. Being shy, he hid behind his father until Lelouch approached him. Lelouch left a deep impression with Rollo and perhaps saved his life as they were playing while Marianne and Lamperouge talked. That's also when Lamperouge guessed Lelouch is his son.
Charles discovered that Marianne met Lamperouge and issued an order to masscre thus wiping out the whole of Lamperouge family leaving Rollo's alive. Rollo's memories may or may not be altered after being taken in by Charles who left him in the military. Rollo undergo harsh training. As time passes by, Rollo had forgotten about family's warmth which made him how he is now. V.V approached him during that period giving him his power of Geass. Because of his linage, despite his superb piloting skills, he may or may not become a member of KoR.
Charles sent Rollo to Lelouch hoping that he will be able to get rid of him after capturing C.C. not knowing that the latter was his half brother. During that one year period, Rollo learnt once again, how a family warmth was like and was suprised when Lelouch gave him a locket as his birthday gift. No one had even treated him that kindly. Hence, Rollo treasured that locket a lot.
That may also explain if Marianne actually bore any hatred towards Charles
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In future, when the Lamperouge brothers discovered the truth behind, they will be able to go all out together fighting against the emperor.