Walternate

Walternate is a Great Being, and one of their best scientists. He survived TPK's purge, and became his chief scientist and part of his inner circle.

He was responsible for TPK's assassin droid programme, as well as the mind control device TPK used to take over the Makuta and force them to merge into one.

It's probable that Walternate has his own agenda, but we have no idea what that might be.