How do I reveal daggers real name?

You can't cause she hide her real name. Her full name is princess garnet till alexandros xvii.

No no no, Not the name given to her by Queen Brahn and the king, her name from when she was a kid in the summoners village, check the eidolon wall after you have given an item to all the friendly monsters and after she cuts her hair. I'm not sure but there might be more to it then that.

In Disc 4, you can talk to Lani in Madain Sari. She'll usually run away, but if Garnet and Amarant are NOT in your party, she'll stay put. Talk to her twice and she'll clue you in about strange writing on the Eidolon Wall that Garnet might want to take a look at. In the room right by Lani you can read an etching that hints at how to reveal the writing -- I'll simplify though.

Put Garnet in your party and head to the Wall. As you enter, start walking left until you're back at the entrance -- you'll hear a "ping" from somewhere. Walk the opposite direction to the entrance and hear another "ping," and keep repeating. On the ninth ping, your HP/MP will be restored; on the tenth, the writing will become available on the wall. I won't spoil it for you, but it's worth the wait.