Aurora Shifter

If an effect says you get one or more {E}, you get that many energy counters. To pay one or more {E}, you lose that many energy counters. You can't pay more energy counters than you have. Any effects that interact with counters a player gets, has, or loses can interact with energy counters.

Energy counters are a kind of counter that a player may have. They're not associated with any specific permanents.

Keep track of how many energy counters each player has. Potential ways to track this include writing theme down on paper or using dice, but any method that is clear and mutually agreeable is fine. (At higher levels of tournament play, dice may not be allowed for tracking counters that players have.)

You don't choose a target for Aurora Shifter's last ability at the time it triggers. Rather, a second "reflexive" ability triggers when you pay {E}{E} this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

Because Aurora Shifter isn't entering the battlefield when it becomes a copy of another creature, any "When [this creature] enters the battlefield" or "[This creature] enters the battlefield with" abilities of the copied creature won't apply.

If a spell or ability with one or more targets states that you "may pay" some amount of {E}, and each permanent that it targets has become an illegal target, the spell or ability won't resolve. You can't pay any {E} even if you want to.