Sensei Wu is the Sensei of all the Ninja who have learned Spinjitzu. He took in all the Ninja and trained them to defeat Lord Garmadon, but also to protect the legendary Green Ninja.
His brother is Lord Garmadon, the Lord of Evil. But he wasn't always evil. He was once bitten by The Great Devourer, and turned him evil. Garmadon has a son, named Lloyd Garmadon, who ended up as the Green Ninja.