Where does the owner *name* for an S3 bucket/AWS account come from?

AWS is stupid here. According to AWS support, it is your root account's forum account's initial nickname. However it is there forever, even later you change your forum account's nickname. AWS is not able to change it.

S3 actually uses AWS root account user's forum account's nickname in the Owner and Grantee fields. There is a discussion about this here.

You can change the forum nickname, in order to change what is displayed in S3, here.