The past is the past, but there is always the future.
Sensei Wu is a main character in both LEGO Ninjago: Masters of Spinjitzu and The LEGO Ninjago Movie. He is a wise old man and the mentor and master to the six Ninja (Lloyd, Kai, Jay, Cole, Zane, and Nya). He is the First Spinjitzu Master's second born son, Misako's brother-in-law, Garmadon's younger brother, and Lloyd's paternal uncle.
Despite his advanced age, Wu as the ninja master is still fit and can hardly be beaten. Before he taught the current ninja, he fought with his brother and elemental armies against the evil in Ninjago, sometimes in the serpentine war or against the masters of time. But Sensei Wu was already had a disciple before his current one: Morro, the master of the wind. The old man had helped him master his strength after finding him neglected by the containers of the monastery, where Morro and a few of his friends had been looking for food. He also always had special feelings for Misako.
Many years later, after bringing the four Ninja together, he taught them the art of Spinjitzu and the handling of the four golden weapons. However, shortly thereafter this monastery was burnt down by the serpentine and since then the Ninja have been trained and trained mostly on a modified ship, the Destiny's Bounty. The Master has been assisting the Ninja throught their many conflicts and adventures.
In a fight with Acronix, however, he was hit by the Advance Timing Blade and aged very fast. Although he could be rescued before the end of the seventh season, he sacrificed himself and disappeared with Acronix and Krux in the time vortex. The ninja, nearly devastated, then appointed Lloyd to be their premature master; because their goal was to find Master Wu; no matter what happens.
Wu continued to battle the Time Twins and was presumably able to defeat them before he somehow exited the vortex. However, the time blades' affects had caused him to regress into that of a toddler. Fortunately, Misako was able to track him down and he's been back ever since, even if only as a toddler. He is also the brother of the cruel Lord Garmadon, both sons of the first Spinjitzu master. His nephew is Lloyd Garmadon, who turned out to surprise everyone as the legendary Green Ninja. He usually wears the staff of the first Spinjitzu master as a weapon. He is in love with Misako, but she chose Garmadon because he intercepted his love letter and replaced Wu's name with his own .... but despite that, Wu and Misako have come closer since the second season. His spinjitzu has a golden color, which corresponds to the power of creation inherited from his father.
Wu is the second-born son of the First Spinjitzu Master and younger brother to Garmadon. A childhood incident led to the latter being bitten by the Great Devourer and slowly infected with evil. When Garmadon left to train with Master Chen, Wu came across the orphan Morro and took him on as a student - which would later turn into his biggest regret. He and his brother were then involved in the first Serpentine War on the side of the Elemental Masters as well as the conflict against the Time Twins. Eventually, Garmadon succumbed to the evil flowing in his veins and tried to take the Golden Weapons, leading to a battle that resulted in Wu banishing him to the Underworld.
Having accumulated much experience and wisdom over the last forty years, he would later seek out the Ninja and become their mentor in order to help them against their foes.
After slowly aging to death, the Ninja were forced to use the time reversal blade on him to save his life. Although they succeeded, Wu was accidentally turned into a toddler because of this, and the Sons of Garmadon kidnap him and Misako in order to use him to get the last Oni Mask. Over time, he gradually ages back to normal, reclaims his father's armor and confronts Garmadon alongside Lloyd.
Sensei Wu, Nya, Dr. Julien & Misako
Wu talking with Nya with Misako & Zane's Falcon
Sensei Wu saved Garmadon's life
Pythor is changed Sensei Wu to evil
Kai, Nya and Wu face the Time Twins in the Iron Doom.