Let's see.. I have had headcanons about the sons of Garmadon for a long time, I guess I can share them.
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-Garmadon hates it when Harumi wears the mask of hatred inside the tower. He's scared that she'll accidentally break something like a pillar holding the place together.

-Garmadon Ofcourse has the whole rooftop for himself. Hence it's his throne room. But as for rooms, Harumi gets the whole ninth floor for herself and the generals(Killow, Ultra, Mr.E) have the eight floor to share. The reason why I chose those specific numbers is because Ultra, Killow and Mr. E were thought to be the antagonists of S8 and Harumi is the second antagonist in S9 after Garmadon.

-The three generals usually have sleepovers with eachother frequently however, they only hung out and eventually leading into a sleepover with Harumi for one night only before she started to distance herself from the others.

-Ultra Violet and Harumi were besties from S8 to S9 ep90. After that episode, Harumi and Ultra Violet kinda had a rivalry between eachother.

-Harumi usually sleeps around 3AM due to her nightmares, Ultra Violet sleeps at 11PM, Killow at 8PM, Mr. E doesn't need sleep since he's a nindroid and Garmadon at 10PM.

-Ultra Violet's bedroom is mostly full of magenta. Her bed is Violet with black flower pillows, there's a vanity somewhere in her room and loads of makeup related posters on the walls. She also has the biggest wardrobe between everyone in the whole SOG crew. It's mostly filled with purple bandanas and loads of punk/gangster jackets. She also has a few handbags. Shiny handbags.

-Killow's room is basically all orange. His bed is black and orange, he has weights and punching bags in his room too. Also, there's a few hooks on the back of his metallic door, for him to hang his bandanna.

-Mr. E doesn't really need a room but Killow and Harumi insisted he got one. He didn't bother to paint the walls Red however, he has a red carpet in the center of the room. His bed is black and red, Harumi got it for him as present. His small wardrobe only contains his current suit but in alternate designs.

-Harumi's room is navy and white. Her bed is navy with green pillows and she has a desk for when she needs to think and draw plans for her crew. There's also two picture frames on the desk. It was the picture taken with her and her biological parents before the great devourer killed them. The other one had a smashed frame and slight dried blood on it(Ultra Violet cleaned it for her when she entered her room one time). It had her and Lloyd together. Inside the cabinets, there were her drawings from when she was still a kid, a picture of her, Mr. E, Ultra and Killow and a picture with her and the emperor and empress of Ninjago. In her wardrobe, there's was just loads of jade clothing and some alternate quiet one palette colours clothing.

-Garmadon clearly stated that he didn't want any green nor colourful colours(just black) anywhere in Ninjago. But just by looking at the four's rooms, they already disrespected that rule.

-Mr. E has a thing for aesthetics. He usually makes some for his friends.

-When Harumi and Ultra first met, Ultra Violet taught Harumi how to jump out of buildings without getting hurt.

-Killow has a slight crush on Ultra Violet.

-Mr. E likes bunnies, much to Killow's annoyance since he is absolutely terrified of them.

-Out Of everyone in the SOG, Harumi and Ultra are the best singers.

-Harumi is about 14-15, Ultra is 18-19, Killow is 20, Mr. E doesn't have an age and Garmadon is around 62.

-One time, Harumi, Ultra Violet and Killow rode the colossus around town, with Garmadon controlling it Ofcourse. That was one of the times where he actually had a laugh with his minions.

-Mr. E is actually a good baker.

-Ultra Violet and Killow like to tease Harumi about Lloyd.

-There's a time that Killow and Ultra Violet went out to a park at night together, just to hang out as 'friends.'. Harumi and Garmadon took it the wrong way and tried their best to ruin their little moment together. Garmadon would usually order his colossus to break their surroundings. Even though most of the night, they had to dodge the colossus, Killow and Ultra Violet had a good time.

-Killow seems to find Harumi and Garmadon's daughter and father bonding moments adorable, Ultra? Not so much.

-Harumi's Hair is always tangled in the morning. Ultra Violet sometimes does her hair.

-Ultra sleeps with her ponytail on at night.

-Harumi considers the three generals to be her closest and bestest friends.

-Ultra Violet usually whines about the fact that Harumi is the 'official' user of the Oni mask of hatred.

-Harumi hates it when the other generals call her: Rumi(Only Lloyd gets to call her that.) or anything that's not: quiet one.
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So yeah! That's my long list of headcanons! More may be coming soon. Tell me what you think please!