Prans evolve through a fairy, child, teenage, and adult forms. In order to advance it, you must complete a quest at Pran level 5, 20, and 50. Your Pran will not gain any more experience until it is evolved.
When it is time for your Pran to evolve, she will wish to speak to you and will give you a quest to speak to Theltar Bariel. He will then give you a quest to evolve it.
The quest to evolve your Pran from a Fairy to a Child requires you to gather up 5 or more fairy waters from an enemy.
The quest to evolve your Pran from a Child to a Teenager requires you to get a Rubicine [F], a Pellurite [F] and a Lesser Auralar(=Raw Auralar). All of the items can be bought from Helen Stadler in Regenshein, or looted from monsters.
The quest to evole your Pran from a Teenager to an Adult requires you to purchase an evolution stone recipe from Auction House NPC Stefano Lantos.
You must collect the materials to complete the stone or purchase them from other players. The recipe will either award you an evolution stone or it may "fail" and award you another recipe with a higher success rate. If you do not succeed in making the stone, you will have to collect the materials again and again until the stone is produced.