Race: Old One, appears as a handsome human Morrigan imagined
Height: 6' (183cm), variable
Hair: Green, wavy and shoulder-length
Kurisu is Cthulhu's son, and has slept deep under the ocean for a very long time. Humans at Pathos-II woke him and began stealing what they called "structure gel" from him, at a source known to very few of those humans. He watched them for a time, finding them mildly amusing as playthings to haunt with nightmares. An "accidental" structure gel leak that had contaminated the water on an invisible, microscopic level several months ago was not actually his doing, but a purposeful test done by those who knew the gel was not all it seemed.
He did not complain about the leak shenanigans, because it essentially gifted him a large area of water all around Pathos-II to do with as he wished. His only major change at the time was to dampen comm-signal strength in the water considerably, making it impossible for suit comms to contact stations without using large-enough robots equipped for communication, or intercoms. Communication between suits still worked, but only at relatively close range.
Some people were less lucky in regards to being Kurisu's mental toys, experiencing delusions or other signs of mental illness. Then one day, Morrigan arrived at one of Theta's top-secret labs, and Kurisu watched as the cute teenager was tortured mercilessly with an injection of diluted structure gel. This displeased Kurisu, and he decided that the teenager would live through the process, something that had yet to happen during the top-secret trials. While Kurisu was brooding and watching the boy recover, that very boy imagined a presence behind the security camera blinking in the corner of the room.
Morrigan made a desperate promise: his soul for whatever sort of salvation the possibly-imaginary presence would offer. The presence was, of course, not imaginary, and later while a doctor was inspecting the arm where Morrigan had received the injection, Kurisu struck. Not only did the doctor die in the process, but structure gel all over Pathos-II errupted with exponential and deadly growth.
Every station rocked with the torment brought to it by walls splitting open and metal bending into new shapes at the will of the gel. Electronic components became tools for the substance too, and many people died. Everyone inside Theta perished, except for Morrigan. Master finally met minion, first as a possessed corpse, then as a visual illusion of a handsome man Morrigan had daydreamed previously. For the moment, Kurisu's human form was still technically a gooey corpse under that illusion.
Kurisu found his minion something of a puzzle, unused to human emotions and especially concepts regarding conscience. There were also new concepts the ancient evil had never heard of, including the concept of outer space. But since he could clearly read Morrigan's mind and view the teen's mental visualizations, it was easy to have simple scientific concepts explained.
Kurisu led the teenager to Theta's residential wing so that Morrigan could find some semblance of comfort there. Kurisu contemplated his next move while Morrigan searched staff quarters for clues on just what the scientists and doctors had been doing.
Pathos-II's communications systems had gone down during the so-called earthquake caused by gel ruptures tearing at the stations, and there were already crafty false messages cached and waiting to be delivered after communications were restored, stating that an asteroid had hit the earth and the surface was expected to be completely annihilated. Surface communications were completely locked out as if to prove the story true.
Kurisu essentially had complete control over Pathos-II at that point, except for the Core which had proved impenetrable after Aster encased it in a crystalline barrier. Aster seemed trapped and useless, Kurisu had taken control over the WAU via its backup systems, and the humans he had paid attention to were easy to kill, so perhaps the surface would be an amusing place to play next...