Takeo Sasaki

Takeo was the son of Professor X and Yui Sasaki. He was a student at the Sasaki institute for young mutants, where he was often picked on by others and was best friends with Hisako Ichiki.

Due to an accident in his childhood, his mother contained him within her research facility next to a large machine designed to prevent others from detecting mutants in the area. She convinced Takeo to use his telepathic ability to wipe everyone's memory of him. In the meantime she worked for years trying to remove his mutant powers while he remained locked away for years.

Eventually, Dr. Sasaki took on a new research assistant Jun Sanada, who would turn out to be the Black King of the Hellfire Club. His ultimate plan being to manipulate the Doctor's relationship with the U-Men in order to gain access to Takeo and use him in order to take control as they had attempted in the past with Jean Grey.

However, Takeo's power proved too much for Mastermind, and after losing control, he was killed when Takeo's powers raged out of control. The X-Men attempted to stop him with little success, but he was eventually subdued when he was once again challenged by Professor Xavier in mental combat. While his body seemingly dispersed into light, he left his parents a mental message, leaving some doubt to his ultimate fate.

Powers
Takeo was a mutant possessing Omega-class powers[1] with the following powers:
 * Reality Warping: Takeo has the ability to easily control, alter, and manipulate reality itself. He has virtually limitless psionic and realty-altering abilities, can easily change states of matter, and has derived numerous powers from this one. He has displayed the ability to:
 * cause snowballs to burst into flames
 * freeze airborne electricity/lightning (which itself is hotter than the sun)
 * transform natural environments like turning snow to crystal or creating volcanic regions where none previously existed
 * bring inanimate structures like floors and pipes to life
 * transform his own body
 * easily control and manipulate space to redirect attacks
 * alter the DNA of other humans, giving them active X-Genes in order to create other mutants
 * cause spontaneous secondary mutations in existing mutants, usually coupled with severe on-set of Damon-Hall Syndrome
 * Psionic Abilities: Takeo displayed the psychic/psionic ability to create a barrier when he was young to deflect attacks, energy blades which could damage Hisako's psionic armor, and can erase memories. He also displayed some astonishingly powerful ability with both telepathy and empathy, attacking Professor Xavier within an illusionary space, and matching him mind to mind.
 * Monstrous Form: Takeo showed the ability to take on a giant humanoid form with destructive tendrils that expanded out across the world that appeared to decay matter.

Weaknesses
Despite his raw power, Takeo seems to possess little in the way of physical or mental fortitude, his body appearing very frail and his mind being overtaken and controlled by Mastermind.

Trivia

 * Takeo bears many similarities to David Haller. Both being the previously unknown child of Charles Xavierand both possessing incredible reality warping abilities.