Kai, Nya and their Parents - The Difference between knowing and not knowing what you're missing - and the sacrifices made by a caring brother
The day Nya was brought home - the day Kai met Nya face to face for the first time; Kai is two (Hands of Time, episode 3, A Time of Traitors)
Three years later
Ray and Maya are kidnapped, thus forcing Kai to take on the responsibility of becoming a five year old Dad aka the death of Kai's babyhood and childhood so as to preserve Nya's babyhood and childhood (Hands of Time, episode 8, Pause and Effect)
Thirteen years later
Kai and Nya are reunited with their parents (Hands of Time, Episode 8, Pause and Effect)
Three years later (vid from Seabound, episode 2, The Call of the Deep)
Though Kai had pointed out that Nya and Maya did not get along (due to Nya's independence from having grown up without her parents); at Master Wu's insistence, Kai calls in his mother to help Nya with her power problem.
Nya, however, dislikes Maya's mothering (what she considers babying).
When Wu suggests that Nya should discuss her issues with Kai, Nya insists with scorn that Kai likely enjoys being waited on and loves to be treated like a baby.
(Note how Nya goes from the probability statement of "probably, likes, being waited on" to the presumption in her absolute statement of "I bet he loves being treated like a baby" -- Nya, please tell me if this is your thoughts on your brother who raised you and gave up everything for you then when has he ever previously behaved in such a way that would suggest your statements to be true.)
The difference here is that Kai remembers what it was like to have someone taking care of him only to have it ripped away - to go from being the baby boy to suddenly having to be the adult for his sister.
Nya, though, doesn't really remember what it was like to be cared for by her parents; but, she remembers what it was like to be cared for (by her brother).
Kai is, in a way, reveling in the joy of being able to play games with his father, eat food cooked by his mother, and learn once more from his parents.
All things he hoped for but never once thought he'd ever have again.
While growing up, Nya was likely allowed to play games, eat homemade food that she didn't have to cook, and learn from Kai the lessons that he remembers their parents teaching them.
Kai on the other hand, had to get straight to work, schedule time in a busy day to cook so that Nya could eat, and be the one to teach trusted lessons from his parents (while likely having to weed out good advice from bad, given to him by whichever random adult decided to give it to him).
Kai's sacrifices for Nya can even be seen in the clothes they wear in the pilot.
When we first see Nya it looks like she's dressed in silk clothes which we know didn't come from Maya's closet since she generally dresses in blue.
Kai on the other hand is introduced in his blacksmithing clothes and goes to train with Wu, in his blacksmithing clothes, which may indicate a limited closet - he may even be wearing his Father's clothes as a way to save money.
But, you might say, he has different clothes in the episode, The Royal Blacksmiths.
Yes, he does have different clothes, but these clothes, like his and the Ninja's matching pajamas (color coded but with the same golden dragon on the chest pocket) are likely the result of Kai now being able to lean on Wu, financially.
Note that Kai's clothes are the only ones that indicate his elemental power - the others likely already had these clothes on hand before they met Wu (except maybe Zane - IDK).
Another way Kai's sacrifices and generosity torwards his sister can be shown is by comparing the first pilot episode to the Season 2 episode 6, Wrong Place, Wrong Time.
In this episode it is shown that, were it not for Nya getting kidnapped by Garmadon, Kai would never had cared about becoming a ninja.
It was only the knowledge that becoming a ninja was the only way to save Nya that motivated Kai to become a ninja.
Yet in a timeline where Nya was never kidnapped, all Kai cared about was getting the blacksmith shop back on its feet after the Skulkin attack.
So how did Wu convince Kai to come to the Monastery, the answer, he didn't - Nya did.
Wu unintentionally convinced Nya to become a ninja and since Kai wasn't just going to let her go off on her own (this is his fourteen year old little sister who he raised after all) Kai went with her so they could become Ninja.
Kai gave up what he wanted in favor of what Nya wanted.
~~~~~
Kai's sacrifices, his giving nature, and everything he does for Nya is almost practically an automatic habit, because he and Nya lost their parents at a young age thus forcing Kai to give up being a child to be Nya's caretaker.
And yet throughout the series whenever the siblings bring up their parents, it becomes clear that Kai made sure that Nya never forgot them and that they were remembered well, that they were missed and not forgotten.
I really like Nya; and while Seabound is a great season it also points out that Nya may just be truly unaware of how much Kai sacrificed for her to the point where she doesn't understand exactly why Kai likes having their parents around; among other details that she and others misunderstand about Kai.
Believe me, I like the idea that Nya knows and helps her brother out where she can, and she definitely loves him, but it seems like Seabound has shown her to be unaware of Kai's trials and tribulations in raising her and keeping them both from landing in the grave of the fireflies.
