Some TMNT stuff really isn't for little kids.
In the IDW continuity story Shredder, Kid is the name of a supernatural entity, whom Oroku Saki describes as a demon child, who is the first figure to greet him in the afterlife that Saki experienced after his death and before his future resurrection by Oroku Karai. His name and nature were then unknown, as is whether his word can be trusted.
He first appeared to Saki when he first arrived in the afterlife and confirmed that unlike the regular residents of that place, the ninja was only passing through. However, the demon child then warned Saki not to disturb the natural order of the afterlife.
Saki ignored the child's warning, and went on to enlist an army of dead warriors to charge on a castle. After he slew their king, the demon child reappeared to reveal that the king was himself an older Saki. He then predicted Saki's future: that he would lead armies and conquer the world, then destroy it and remake it; wipe away humanity and build a new world full of barbarians and mutants; and eventually die and become the old king, only to be slain by his younger self. The demon child seemed to be amused by this prospect.
Kid reappeared in the mini-series Shredder in Hell after Oroku Saki's second death at the hands of Saki's clan brother Hamato Yoshi's reincarnation Splinter. As Saki kept addressing the figure as "demon child," he told Saki to call him "Kid." Kid revealed that he is a "low entity" and master of Oroku Saki's afterlife realm, but has no knowledge or access to other supernatural realms including Saki's dreams. Kid also revealed that, by coming to Saki's aid, Yoshi had forfeited nirvana.