LIAM.
BRADY'S CLOSEST FRIEND, LIAM. The one he spends most of the time with during practice and playing games with. The boy that Aubrey trusts her brother with to keep him safe and protected happens to be related to a god.
She isn't just jumping into conclusions blindly; she remembers clearly the events of that night at the club. Chad had acted so cool with Kai's sudden presence even when the other six rich kids were all unsettled; also, he and Kai happened to know each other well to be on first name basis; however, the major incident that triggered Aubrey's knowledge that night was the fact that after Kai paralyzed everyone in the club, Chad was no where to be found.
Connor previously mentioned that there are gods -more than one could count- who were banished down from Mount Olympus so she would not be surprised if Chad happens to be one of them. But Liam can't be one of them, hopefully being his cousin doesn't mean anything.