You'll never remove the last counter since it's phased out, so all creatures will remain phased out indefinitely
For those unfamiliar, the term aggro is short for aggressive, and in this case, refers to a strategy that seeks to put lots of low-cost cards into play and gain enough of an advantage to win before the opponent can stabilize
If the card keyword read chosen characters without the apostrophe, it could mean add to multiple characters
But, Jace is still the best way to switch from an even position to a game winning one, and its very difficult to win the game when your opponent has brainstormed a bunch of times
Free spells are very, very powerful, and Id argue its between this and Fierce Guardianship for the best in the cycle
One of the main challenges in Magic is fighting against the randomization of the cards in your deck, and an easy-to-cast tutor like Green Sun's Zenith effectively removes that hurdle