@gabrielvillenet It's not saved in any Opera's database. When you set up a passphrase, your synced data is encrypted locally, before being sent to the Sync servers and that's why you can't access your data at www.sync.opera.com.
When you enable Sync in another installation/device, you need to provide the passphrase in order for Opera to be able to decrypt the synced data and use it.
If you don't remember the passphrase, Opera can't decrypt the data and it can't be used.