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From Brickipedia, the LEGO Wiki
The ninja in NINJAGO have had a range of several different suits. These different suits can indicate that a ninja has achieved a certain rank or status, or have a more practical reason such as protection from a certain type of enemy. The original four ninja (Cole, Jay, Kai and Zane) received their original trainee ninja robes in the original 2011 NINJAGO wave, while Lloyd's first set of robes were released a year later, and Nya got hers after becoming the Water Ninja in 2015. Additionally, Skylor was released with Jungle robes matching the other ninja during Chen's Tournament of Elements despite not being a member of the team.
