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I wonder if the person who gives the Geass influences what it will be, or if it's somehow tied to the receiver's personality and needs? It seems like the latter... I'm not fond of C.C. but she's not the type of person to intentionally give a power like Mao's. I think there's more to Rolo than random orphan - there would have to be a reason why he'd want to stop time. And his resemblance to Nunnaly does suggest that they're somehow related... perhaps an illegitimate child? Also, a power like Rolo's would be very useful for setting up Marianne's suspicious murder.