Since a few people have asked about this, here's how the crazy energy system works.
First, you don't need a grimoire to learn a spell. They're only for reference.
Second, when you read (use) a grimoire, it adds it into your list. You can see that in the 'other' menu slot (says DIR) on the far right of the menu, second button there being grimoires. It's in a different spot in battle.
When you select a spell's name from the grimoire list you'll see arrows above the heads of each character that can learn it. Note these only update properly when you select the spell or flip a page (bug w/ original game).
Most characters learn spells when they 1) have enough energy to use it and 2) they attack and/or kill something. If possible, they immediately use the spell that was learned (watch out when learning tornado!). Again, you don't have to have a grimoire for a spell to learn it. Once learned you can reallocate your power without forgetting the spell, although you won't be able to use the spell unless you meet its requirements.
Most spells call for different kinds of energy, some positive and some negative. Every time you level up you can add one more block to an energy bar and get one more bit of power to shift around. You'll want to check your grimoires before leveling up to see what spells a character can learn and tailor the energy levels to build toward it.
Star works a little differently. He learns spells when he kills specific enemies that use them, usually the first guys you meet who use them. Not certain if he has to kill them with basic nail attacks or not, but he must be the murderer.