There are various items you can use to strengthen your Heroic Spirits. Radiant Stones and Magic
Stones can be used to enhance their skills, while Pieces and Monuments of their respective class
can be used for ascension. In order to strengthen them to their maximum potential, rare materials
obtainable only from specific monsters and special quests will be required.
This game is quite bothersome, huh?
That's because you only care about summoning, you should at least raise them properly as well.