Baal: Complete Extra Map 9
Etna: Complete Extra Map 2
Flonne (Fallen Angel form): Complete Extra Map 2
Laharl: Complete Extra Map 4
Mid-Boss: Complete Extra Map 5
Myao: Complete Extra Map 8

To unlock Bottlemail, when you have enough items including trees, plants, skulls, weapons, and mushrooms, go to the large tree located by the roof of Marona's house. Put a stone on the middle of the house to make it easier to jump back and forth from the roof to the tree. Carry an item and throw it on the tree. Keep piling up items to make a large tower-like object. Once you have a very large pile, climb to the top. If you cannot reach the top, go to a larger tree that is too large to jump on to and make a smaller tower that will allow you to jump onto the place where you cannot make it. When you are back on the big pile, jump to the very top and jump a few times. If nothing happens, you must make it bigger. To know if something happened, a small screen will appear stating that you unlocked the character [random name]. When you go to its status, you will see that it is Bottlemail. If you add on to the tower and jump to the top, you can get 300 gold.