By default, the each Droplet with IPv6 enabled will be configured with a single IPv6 address. This is the address that will be used for any PTR records that will be generated for domains pointing at your server. Like with IPv4, if the hostname given to the Droplet is a FQDN, the PTR record will be generated automatically.
If you're looking for more control over PTR records more generally, there is an open request for allowing users to manually set PTR records. Please vote for it if that's something you'd like to see: