Mana. It's not blue! The second meter and resource your character uses. Mana is used to when you cast a spell, to maintain certain spells and songs, and for Charged Clutch.
Mana regeneration rate is based on your Mysticism and your current vigor as well as if you serve Princess Kateriina. Certain spells, like Jala's Rejuvenate or Faren's Mana Focus can increase your mana regeneration rate, while Focused Mind decreases it. Additionally, you can cast Meditate to restore mana, and skilled Bards can call upon Crystalize Mana when they need a mana boost.
Certain mobs (here's looking at you Xeochicatl) can drain your mana when attacking you, so mages beware! Fortunately, those learned in the ways of Sorcery are immune to some forms of mana drains, as well as those trained in the art of Crystalize Mana.