Clarity

That was my guess from the very beginning! Spike was standing among the mane 5, when Celestia guessed that Twi would need the help of all of her friends to find the keys.

I hope poor Spike hasn't gotten his fill of episodes yet, because he really was a background pony in the last few episodes, as with AJ and Rarity.

Eeyup. I honestly hope that he'll not be shoved into the background in the next episode. That potential...