Keyblade
The main weapon in Kingdom Hearts, the Keyblade is weilded by Sora and is the only thing that can stop the Heartless from taking the hearts of planets.
It's used to lock the keyholes behind which the planet's 'heart' is, so Heartlesses cannot get to it. It's also the only thing that can get rid of Heartlesses entirely.
It's upgradable via keychains, each of which change it's appearance and powers.