Here's a bit about Casey! :D

Their last name is Elsher, but almost nobody knows that, because they simply refer to themselves as Casey.

Casey is very secretive about their past, so none of the ninja know their real name, which puts even more mystery around them.

They have a very gender-neutral apearence, which is why their gender is more of a mystery between the ninja, and is often a topic in conversations when Casey isn't there. Their short hair and the fact their chest is suppressed most of the time doesn't really help either.

Kai and Cole think that they are a boy, while Jay is absolutely and utterly confused about their gender, similarly to Wu. Zane thinks that Casey is a female, but uses they/them pronouns for them. Nya mostly thinks that Casey is a girl, but doesn't completely discard the possibility of them being a boy. That doesn't stop her from hanging out with Casey and calling it girl night, though.

Casey is the purple ninja, Master of Flora.

Flora is the power to control plants. Take Willow from The Owl House as a reference, because it'll look something like that once Lloyd shares his powers with the ninja.

Flora is one of the secondary powers, which is why it isn't needed for the Tornado of Creation.

The Kusarigama of Flora was created as an extra, just in case something goes wrong, and is not necessarily needed, which is why Garmadon said that he'll be able to hold the four weapons, despite there being five.

Wu only picked up Casey because there were five Golden Weapons.

Casey's dragon is Lamium, the Flower Dragon. Why is it "Flower Dragon" and not "Flora Dragon", you may ask? Because Flower sound better.

Casey is very bad at remembering names, which is why they have nicknames for everyone.

Kai is Hotspur (because of his hot-headed personality).
Jay is Spark, Sparky, Sparks (reffering to his power).
Cole is Fearless (because of his "i don't fear anything" attitude. yes, i took that from tmnt, bite me.).
Zane is Bear (taken from "polar bear", reffering to his power, and because he's built like a bear).
Nya is Captain (because she's mostly controlling the Bounty), Ma'am (when talking to her directly).
Wu is Sensei.

They are the most irresponsible out of the ninja, and cannot be trusted in the kitchen without supervision.

They only use the kitchen to make cookies, which they always offer to the postman when he comes, and they call him Mr. Mailman.

They always have sweets on them, despite not liking sweets that much.

They have their own room, which is filled with plants.

A little bit of facts in previous episodes:

Way of the Ninja
While entering the monastery, Casey was suspicious of Wu being actually asleep. They never found out if he was awake the whole time, or woke up because of the fight.

The Golden Weapon
Jay's "Four leafed clover. Four seasons." line is originally written by SavannahNinjago. Go follow her.
Casey speaks with Sensei pretty casually, but still respects him, as showed by the "'pologies" line.
The day they met Sensei, they were following a map which they got from Mystake.

King of Shadows
Casey's "so less needing me, hooray" comes from the fact that they're not really involved in the creation elements, so.
The "of course it was you, Hotspur" line was also sarcastic.
In Wu's "my brother must not unite the four weapons" line, he said four instead of five because four weapons are enough when combined. And by four, it's meant the canon weapons, not Casey's Kusarigama.

Weapons of Destiny
Casey was absolutely ecstatic to ride a dragon.

Secrets of the Blacksmith
Casey sometimes just plants flowers around wherever they are. That's why they were with the citizens.

Flight of the Dragon Ninja
Every time they have those competitions, Casey just goes and does their own thing. Everyone got used to it and just doesn't question it anymore.

The New Masters of Spinjitzu
Their Kuragisama was actually in the ninja's room, because they forgot it there. While Krunch and Nuckal were doing their thing, Casey was awake, taking care of their flowers.

An Underworldly Takeover
Casey has been following Cole around the whole day, which is part of the reason they were there to ask Wu. The other part was curiosity.

Return to the Fire Temple
Casey started asking Kai questions the moment he came back from the Fire Temple.

Battle Between Brothers
While Wu was talking, Casey was actually outside of the door, listening.