Cupil is first seen flying out of Fina's sleeve. Fina introduces him as her dear pet that kept her company ever since she was a little girl. Cupil makes "kyu" sounds while Aika comments on how cute he is. Fina explains that Cupil takes the form of a bracelet around her wrist when she's not in battle, but Cupil can change into different shapes to fend off enemies.
Cupil is defined as a weapon/character. He primarily comes in a silver color because of Fina's silver moon stone, but like every weapon, Cupil can change into different colors depending on the moon stone used. His strength will depend on the strength of Fina. He just floats around Fina's head when not in use. Although, Cupil does appear in Fina's S-moves, most prominently in Lunar Winds where Cupil sucks up air and unleashes it on enemies. Cupil will probably end up being the weakest hitter in the party, but strangely enough, when Cupil reaches his final transformation, Final Cupil, he will end up being the most powerful weapon in the game...
Cupil can change forms when he is fed enough Chams. Chams are tiny moonstones that Cupil can eat, and they can be found in many different areas. Cupil can also eat three Abirik Chams, which are special types of Chams, but those should only be fed to Cupil when he is in the appropriate form. Luckily, if the player makes a mistake, the player can always get a hold of a "Chom", a contaminated form of a Cham. If Cupil eats one of those, he will regurgitate all the Chams and Abirik Chams that he ate. Then the player can re feed him the Chams and Abirik Chams in the appropriate order. It probably sounds disgusting, but Cupil doesn't seem to mind it.
Here is a list of forms that Cupil will take (only the first and last form is in the proper place because there are different routes a player can take when feeding Cupil Chams or Abirik Chams):
- Cupil (default form)
- Cupil Blade
- Cupil Cone
- Cupil Cannon
- Cupil Club
- Cupil Sword
- Cupil Lance
- Cupil Star
- Cupil Spike
- Cupil Cutlass
- Cupil Hammer
- Cupil Pan
- Cupil Weight
- Cupil Spear
- Cupil Claymore (at least twenty-four chams fed to Cupil)
- Final Cupil (all thirty chams and all three abirik chams fed to Cupil)
A player can be extremely tempted to refer to Cupil as an "it", but Cupil is probably male because that's how Fina refers to him. Cupil probably has a habit of inhaling things, as seen in Fina's Lunar Winds S-move and his attack animation when he is Final Cupil.
When the player establishes a base on Crescent Isle, Vyse can ask Kirala to remodel a fountain "in praise of the moons" for five thousand gold. When Vyse returns to see the finished fountain, it features a rotating stone Cupil that spits out water.
When Fina is severely damaged, Cupil can be seen floating tiredly around her head as though he is severely damaged as well despite Fina being the one that gets attacked by the enemy. When Fina dies, Cupil slowly falls towards her and presumably takes the form of a bracelet around her wrist.