The Venus Climate Orbiter is also known as Akatsuki and Planet-C. Why is a Venus spacecraft called Planet-C? Venus is the second planet from the Sun, but C is the third letter of the alphabet.
An ISAS mission in development is known by a series and a letter. After successful launch, it gets a name, based on suggestions from its developers. One I worked on, ASTRO-E, unsuccessfully launched, was rebuilt as ASTRO-E2. Once successfully launched, it was named "Suzaku", after a mythical bird that is reborn in some stories.