You can win the rare Namingway Card from Mario during the card tournament in Treno. You can also find the card inside Kuja's room during the sequence in the Desert Palace by searching behind the stacked books in the foreground.

Take the card to Daguerreo and find the man roaming the library on the mid level. He asks to see the card, and then becomes inspired to create new names for your playable characters. You can now ask this person to rename any of your characters, including Zidane!

To reach the mid level, examine the strange block next to the Medicine Shop. This releases an obstruction. Speak to the scholar near the bookshelves at the right of the room. After finding the book he's looking for (in the right of the room), speak to the scholar again. This causes him to move from his current location. You can now access a hidden ladder that takes you down to the middle level.