Nya is Kai's younger sister, and the daughter of Ray and Maya, their parents. Kai and Nya didn't see Acronix and Krux take their parents when she was young, thus this is her only family. Because of this, Kai is overprotective of his sister as he wanted to keep her safe from danger. It was this side of him that caused Kai to decide to become a ninja so he could better protect her, and earned his true potential doing this. Nya is Samurai X during this time, but then became a Water Ninja, in which she inherited her mother's elemental power.
Ray is the former Elemental Master of Fire and part of the former Elemental Alliance. Kai's blacksmith and old school abilities were inherited by his father, including the Elemental Power of Fire. He was taken away under Acronix and Krux to work.
Kai and Nya eventually found him in the Dragon's Forge and were reunited. Later, Ray said that the fourth Time Blade was in the Boiling Sea. While the other Ninja fought, Kai and Nya fought the Time Twins and then Acronix uses the Time Punch on Ray. After traveling back in time, Kai and Nya undid the Time Punch using the Reversal Blade.
Maya is the former Elemental Master of Water and part of the former Elemental Alliance. Nyas's fighting and old school abilities were inherited by his mother, including the Elemental Power of Water. She was taken away under Acronix and Krux to work. During the Return of the Time Twins, Kai and Nya found her in the Dragon's Forge and were reunited. Later, Ray said that the fourth Time Blade was in the Boiling Sea, and Maya told Kai and Nya to take the Fusion Blade While the other Ninja fought, Kai and Nya fought the Time Twins and then Acronix uses the Time Punch on Ray and freezes the others. After traveling back in time, Kai and Nya undid the Time Punch on Ray using the Reversal Blade.
Kai and Skylor - Compatible
Skylor is the Elemental Master of Amber and Kai's current "wife"/love interest. He first met her on Chen's Ferry to his island. From the moment they met, Kai immediately fell in love with her right after he got his first look at her and developed a romantic interest in Skylor. During the Tournament, Kai grows to trust Skylor, especially when she shows information regarding Cole and Zane. He was shocked to learn that she is Chen's daughter and was secretly working with her father to undermine his friends, but there is a conscience in her. After the Second Serpentine War, their relationship comes back together, and Kai offers a spot on the Ninja team, but Dareth interrupts them, and also, she would keep in touch with Kai.
Wu is the Ninja's mentor and the current Elemental Master of Creation. Kai is Master Wu's student, along with the other five Ninja. In the beginning of the series, Kai was recruited by Master Wu to become a Ninja. At first, Kai didn't care about training with Wu, but as the series went on, they grew closer. Kai and the other Ninja cared about their master and is glad to have him mentoring them, even though they train hard and get asked hard questions.
When the Ninja found out Lloyd was going to rule Ninjago, they stopped him and Lloyd became the Green Ninja. When he redeemed, Kai realized his destiny was to protect him, thus earning his true potential. Though Kai was jealous he wasn't the Green Ninja, it didn't hinder their relationship.
Cole is the current Elemental Master of Earth. Kai and Cole are loyal buddies on great terms. They share several common things: their confidence, courage, anything along these lines. In the earlier seasons he and Cole were the leaders of the team and sometimes they alternate over the leadership. In Season 5, the Ninja fight over who gets to be the leader.
Zane is the current Elemental Master of Ice. Like all the other Ninja (except Lloyd and Nya), Kai initially thought Zane was weird, but when he found out that Zane was a Nindroid, the other Ninja supported Zane as their brother. He and Zane grow closer throughout the series. When Zane sacrificed himself to defeat the Overlord, the Golden Master, and was thought to be destroyed, Kai was greatly upset over his friend's demise that he couldn't be one of him. But when they reunited, everything went back to normal.
Jay is the current Elemental Master of Lightning. Jay and Kai are loyal friends on good terms, though they interact only occasionally throughout. They do truly care about each other like best friends. At first, their status was generally tense and turbulent since they were fairly distant, rarely interacting. Their status became even more strained after Kai's sister became enamored with Jay, a love circle formed between him, Nya, and Cole, and much to Kai's disgruntled chagrin. However, he soon respected their relationship.
Akita · Alan Parrish · Aspheera · Char · Cole · Colten · Cyrus Borg · Dareth · Faith · Garmadon · Jay · Kai · Kataru · Lloyd · Misako · Nya · Pixal · Ronin · Scales · Scott · Skylor · Unagami · Violet · Wu · Zane