Shai Eisenhardt-Kitumaini (he/him) is the youngest of the Magnet family, and absolutely made all of Magneto's other kids very bitter, because Magneto absolutely loved that chubby-cheeked little fish baby and his little webbed feet… And none of them ever got that kind of treatment with their snappy, curmudgeonly father. (Most importantly, Pietro struggled with Shai's existence for many years.) Magneto's gift to Idira, Shai was the final addition to his bloodline (with Magneto and Idira's knowledge in genetics, Shai was a test-tube baby) before he was buried about 13 years later. In the world, the mutants sort of won. They can mostly live in society without too much fear from humanity at large, but the Friends of Humanity and other Anti-Mutant groups definitely still exist and are very active.
Magneto took it upon himself specifically to train his fish-boy son to the best potential he possibly could, despite being quite old at this point. Shai grew quickly, matching his retired-Terrorist father in stature and height, if not a little bit wider. It initially seemed to be that his ability to quickly adapt to using his mutations was simply an extension of his Fish Person Physiology and perhaps the X-gene he carried from both Magneto and Idira. …Though Magneto never got to see his actual X-gene ability: Mutation Empowerment. In times of intense stress and danger, Shai can transform his body into a much more monstrous fish-monster to deal with tasks at hand. Iron-hard scales deflect bullets and punches, his muscles and hands morphed into gigantic arms and fins and claws. His mouth, while his teeth already regenerated quickly, grows many, many rows of teeth in addition. He later learns more about his connection with the water and ocean that's inherent to his being with his living father, Idira.
Shai's fathers instilled in him a great pride and appreciation of Mutants and their abilities. He learned of Idira's culture, and is regular attendee at a local Reform Temple. He's quite invested in religious study in his free time, even following Kashrut, despite his father's mostly-non-involvement with the religious aspect of his history. Embracing who he was, and the reality of other Mutants, no matter their appearance, was encouraged.
Shai ultimately is very socially awkward, not really sure how to approach common high-schooler social problems, but has a strong sense of justice and equity. He also knows he's 6'10" and built like a Mack truck. He smiles like a kindergartner who doesn't really understand the concept of photographs, he cries loud and often, he's just a sensitive and very emotional boy. He does carry the stigma of being the child of the late Mutant Terrorist Magneto, but at the end of the day, he is a kinder evolution of the man that passed on to continue to fight for Mutantkind's rights and dignity.
The late Magneto himself spent many years with Idira Kitumaini and enduring trauma therapy with Jean Grey (which was terrifying for Jean and harrowing for Magneto) as he worked for the Xavier School as a particularly asocial and cranky Administrator, which was surprisingly a good menial job for him due to his history with multiple Doctorates and known languages, and his past of managing not only organizations but also countries. And yes, Charles and Erik fought vocally weekly, and still sat around in the morning like old men to play chess every week and discuss the books they read at home. Not focusing on his pain and taking himself out of danger helped his mind to be eased more, especially with work he and Charles had done together to stabilize Humanity's opinion of Mutants and greatly disenfranchise the organization of hate groups like the FOH. They definitely agreed on almost nothing, still. Even about the amount of sugar in your coffee, what a proper balanced meal consists of, and the best over-the-counter pain medication.
All of that culminated in that big guy of a fish, Shai! A (slightly) more healed Magneto who taught him to empower himself, and another traditional and culturally-minded and kind father taught him everything he knows. Shai is observant, a sweet boy, and though confused by social convention, with time he becomes an impactful public speaker and interfaces well with the public. Also, you probably don't want to do hand-to-hand combat with him, though he has moral objections to ever killing anyone. He's a great friend to all! As an adult, he comes to lead the X-Men as the pro-Mutant activist (and mercenary) group it becomes after the deaths of Charles and Erik.
Shai is somehow both simple and complex. He's figuring himself out, too!
