This post will contain some of the headcanon I've developed for Kid through both roleplaying and writing Kid-centric fanfiction. Sometimes I'll also draw on other fandom interpretations. This is a permanent WIP that will change as I grow and learn the character.
Little supernovas in my headLittle soft pulses in my dead
"I’d forgotten because he’s usually so low-key,
but he’s a shinigami after all... that was a whole different order of magnitude.
The Heir
Kid is his father's son through and through -- and he belongs to no other. Death gods don't have children in the manner of humans, but instead are fashioned directly from their parent's soul and then given a physical form. As such, Kid does not have a mother and his bond with his "honorable father" is extremely strong. As he stands to inherit the position of Grim Reaper after his father, he takes his duties very seriously and is naturally protective of human souls regardless of what world they originate from.
→ Inspired by this fanfiction: "Kid was the son crafted from his own soul and certain things were supposed to be inherent to him: the mastery of souls, the skill to wield any Weapon if chosen, the tongue of languages both living and dead as they pertained to his duties."
Speaking In Tongues
Kid speaks basically every human language, whether current or "dead" (no pun intended) but is especially fluent in the ones that his father's lifespan encompasses.
→ Inspired by this fanfiction: "Neither liked to mention it much, but [Shinigami & Kid ] were both rather fluent in all languages -- death came to all, no matter how they spoke."
Little souvenirs and secrets sharedLittle offguard and unprepared
"A healthy soul resides in a healthy mind and a healthy body."
Universal Meister
Kid's choice to wield the Thompson Sisters in their Demon Gun forms is both a matter of preference and necessity. Due to his extreme OCD dependence on symmetry, he refuses to use any other weapons. However as a shinigami, he is physically capable of resonating with any weapon at hand and only a psychological barrier prevents him from doing so.
Konso
As a shinigami, Kid is able to see and manipulate souls.
I can see the patterns on your faceI can see the miracles I trace
"Quote."
Title
text text
→ Inspired by this fanfiction: "That was how he balanced himself against his loved ones. He always demanded so much from Liz and her sister - he needed so much from Liz and her sister - but in return, he devoted himself to them with the same relentless, obsessive drive that dictated everything in his life."
Symmetry in shadows I can't hideI JUST WANT TO BE RIGHT BY YOUR SIDE
"Symmetry is my ultimate aesthetic."
A huge part of Kid's personality is dictated by his obsession with symmetry; it literally infuses every aspect of his life and perception. It's speculated that Kid has either
OCD or
OCPD, or a combination thereof. It's not simply that Kid wants things to be neat and organized, but that he needs them to be for him to properly function or even interact with it. Even the smallest detail can literally send him into a panic: some common canon examples are the paintings hanging in his mansion are aligned by the millimeter, and the candles are constantly resculpted as they burn down with wicks are trimmed to an exact length.
It's hard for most people to grasp how deeply ingrained this disorder is for Kid, not understanding that it's not something he can simply push aside given the situation. When Kid is exposed (an accurate term) to asymmetry he literally loses control of himself, experiencing alternating revulsion and panic attacks, coupled with the overwhelming urge to correct the "wrongness" of it. The Sanzu lines in his hair are a particularly devastating feature for Kid because despite his best efforts to be rid of them, they're resistant to human means of dyeing and trimming. When they're pointed out, Kid's self-confidence is immediately destroyed and he'll refer to himself as "worthless trash" and other epithets. In extreme cases, he'll have seizures or explode blood and pass out.
→ Examples of this behaviour can be seen in this episode between timestamps 3:50-5:55 and again from 20:20-21:15.