(A thread where I come up with new mutants, specifically for Arakko/Amenth. Others are free to contribute.)

Ter En Ki, Stopper of Hearts
While appearing older than her brother, the Stopper of Hearts is the younger child of the Brass Bull, and while her mother exults at her dangerous mutation, she despises the mother who abandoned her at birth, and only seemed interested in claiming her as daughter when she developed her mutant power to control the surface tension of liquids to clot the blood in a living body and kill people with a touch. Originally, she was a normal child, abandoned to the appointed caregivers, those whose mutations proved useless for matters of war or survival in the harsh lands of Amenth, and when she reached puberty, her power at first seemed little more than a pointless trick, to cause fluids to become firm, with a consistency between leather and gelatin, such that she could walk over temporarily solidified water. But when she learned how to focus it on the liquid in a person’s body, causing their heart to fail as it tried to circulate blood now thick as soil, she gained new recognition and was sheparded from the work enclaves for ‘useless’ people.

Ter En Ki is a lean woman with a long strong ‘mannish’ face, and any pretense at beauty would be spoiled in any event by the lackadaisical care she affords for her hair, usually bound up in careless dreads, or attire, which seems to be whatever she thought was clean enough that day. She has a borderline obsessive fascination with the nature of her powers, stemming at least in part from a fear of being thought useless or weak and being shunted back to the camps, and so has turned her powerful mind and will to developing new techniques for her gift. One line of study has taught her how to momentarily ‘stiffen’ the water in her own skin cells, so that her skin becomes very hard to penetrate, such that a sword slash only slices open the very top layer of cells, but cannot penetrate the solidified water within, leaving only a thin red stripe, as if scratched by a prickly thorn, and not a metal blade! Another notion led to the discovery that she could affect not just fluids, but even some other substances that assume fluid properties, such as sand, and she can ‘stiffen’ loose sand so that she and her allies could run across it effortlessly, and allow it to ‘soften’ behind her to hinder anyone pursuing her. She has a keen interest in learning new applications, and the scientific knowledge now available to the Arakki, after so many generations 'stuck at the bronze age,' intrigues her, making her one of the first Arakki to show interest in learning new scientific theories that might prove relevant to her powers development, particularly in subjects like fluid dynamics. Due to her estrangement from her mother, she prefers to use her sobriquet Stopper of Hearts, than her given name, although she is still pondering a more distinctive moniker, like Slows-The-Blood or Blood-Runs-Thick.

(Someone whose mutant power is to walk on water? Yes, and I made it vaguely useful! I wanted something other than 'throws zaps' or 'animal person mutant' or 'telepath #42,' and controlling the surface tension of liquids sounded pretty darn out there.)