The Realm of Kokuraimaru is a huge, wide, endless plain covered with countless craters, gashes, and burn-marks, while also being shrouded in darkness, as it is always nighttime there. There is a faint, eerie glow however, due to there being a group of extremely dense storm clouds present in the sky, from which high voltage lightning comes out to scorch the ground. There are also strong and violent gales that constantly blow in this inner world.
The storm clouds almost always have in them thunderstorms that have black lightning with a dark-blue outline. That black lightning within the inner world is itself Kokuraimaru. He is everywhere within this realm, as an almost omnipresent being.
As he is made purely of lightning, he is actually able to take virtually any form he wants (note: this ability does not
translate into Shikai or Bankai; it is Kokuraimaru's ability alone). Whenever I go into my inner world outside of Bankai, Kokuraimaru would take his lightning form out of the other places and present most of it in front of me temporarily in the form of free-flowing lightning (for the amount of time that I am in the inner world).
He was not always a Zanpakutō Spirit. Originally, he was Raijin, the Japanese Shinto god of storms, thunder and lightning. He "died" and later reincarnated as my Zanpakutō with some help from Ouetsu (creator of Zanpakutō) in the process of becoming my Zanpakutō, according to what he's told me. He has a companion wolf made of the same dark blue energy as he is, with his body wrapped in lightning. His name is Kokuraiko ([黒雷子] Dark Lightning Child) and he is an incarnation of Raijū
, who is said to be a companion of Raijin in Japanese mythology and Shintoism. Whenever black thunderstorms are occurring, he gets agitated and creates trouble within this Realm. Kokuraimaru has to flex his reiatsu to calm him down. Sometimes, when that is not enough, I have to weigh in with my own reiatsu.
In character, Kokuraimaru is kind and strong-willed, but is sometimes easily angered. When we get into arguments, there are times when he tends to get a little cranky when he is unable to get the last word and/or "win" the argument. He also has a very bad habit of coming off as arrogant without meaning to, oftentimes. Kokuraiko is the same in personality as Kokuraimaru.
When we spar, he takes the form of a man wielding his Shikai sword. When I visit the inner world in Bankai, there is a noticeable change in the inner world to represent that: the complete lack of black thunderstorms due to the fact that, in Bankai, he is permanently in his "human" form, as a master swordsman Spirit that is 6ft in height, has almost no weight due to being lightning, and blue eyes and blue-black hair that reaches down to his waist, hanging loose (untied).
In his "human" form, he is dressed in a samurai get-up that is enough of a darker blue that it looks almost black, except he is barefoot. He wields the same sword I have in Bankai. The belt on the outfit is black, with the kanji for "black" on the left strap of the belt tie and the kanji for "lightning" on the right one. Kokuraimaru is ancient himself, but his "human" form looks to still be in his 20s, and is also seemingly lean and well-built. On his back, with their top sides facing forward, are three black drums with three tomoe on each of them. There are no drumsticks to go with the drums, but they are not needed as Kokuraimaru is able to beat on them with just the force of his reiatsu alone (is translated into Bankai as well; it's not that I do this with my own reiatsu, it's still Kokuraimaru (no Kenshi) himself who does). When Kokuraimaru is in this form, Kokuraiko likes to sit either on his head or on his right shoulder.