In Witch Spring, unlike its successive games, Pieberry has a large range of freedom in her choices. There are no clearly-defined chapters, and instead your decisions on what Pieberry does in the game's 100 days decide which of the nine possible endings you acquire.
This guide will focus on attaining the "A Peaceful Day" ending.
Completing all of Justice's story will unlock the "Hope given by love, Love given by hope" ending (which is the "true" ending).
Every time you defeat Justice, pick the option to let him go.
In order to meet Luna, you must first visit Elyicion Temple; that is the only way you can enter the Icy Field, where her house is located.
Completing Luna's story will not yield a special ending; however, if you complete it and get the "A Peaceful Day" ending, then a special version will play that includes Luna.