Renata has "a powerful shield against physical attack," with the ability to distract people who come towards her so that they end up going the wrong direction. This is probably because either she was a hardcore coward in her human life,
she is a weak Meyerfemale or she has found a way to make ADD contagious. Her job in the Volturi is to follow Aro around, though whether this is because he actually needs the help or simply because he likes having vampire girls follow him everywhere is not entirely clear. Bella says in Breaking Dawn that she could beat up Renata, since, as with absolutely everything in the Meyerverse, her power doesn't work against miss special snowflake. This is, however, Bella's judgment, so you may wish to take that with a grain of salt.
Special Meyerpire Power Edit
She can put up a shield that makes you forget what you were doing, so you wander off forgetting that a couple of seconds ago you were lunging at her with fury.
She also Aro's bodyguard. She always stands next to him, it could be a sign of an affair between the two.