Origin and history

Onyxx (Sidney Green) was born in an unnamed town in New Jersey.
As a child, Green found himself passed between various foster homes and orphanages.  Awkward and constantly rejected by his peers, matters were only made worse by a case of chronic bed-wetting.  Bullied by fellow children and even adults who were supposed to take care of him, Sidney wallowed in misery until his mutant powers manifested.  Large and strong, the bullying--and even the bed-wetting--seemed to taper off.

Shortly after his powers emerged, Green joined the Xavier Institute where other teenagers like him trained to become the next generation of mutants.  Mentored by the then-X-Man Gambit and taking up the codename Onyxx, he would join and train with a squad of other physically mutated, urban teens.

Bizarre Love Triangle

Onyxx got along well with his squad, most notably with his teammate Bling!.  When the group gained a newcomer by the name of Foxx, Onyxx developed a dangerously intense crush on the new girl.  When Bling! revealed to Onyxx that Foxx was really longtime X-Men foe Mystique in diguise, Onyxx felt immensely betrayed and confused and attacked Bling!, accusing her of lying. 

He then snuck into her prison cell and tried to force her into shapeshifting back to Foxx and say she loved him.  When Mystique callously rejected the young mutant, Onyxx tried to attack her but she quickly slammed a toilet into his skull, leaving him in the infirmary for days.  Onyxx now has a permanent crack on the metal exterior encasing his head from the injuries he sustained.

Decimation

When the Scarlet Witch magically removed 90-98% of the mutant population's powers in what is commonly called M-Day, Onyxx was one of twenty-seven students to retain their powers. Emma Frost ordered the students to participate in an all-out brawl to determine who would compose a sole training squad devoted to a crash course in combat and defensive skills.  However, Onyxx was not one of the seven students to make the cut.

Shortly after the brawl, the school is attacked by the mutant hating Purifiers, who were exploiting the condition the mutants were in.  After several students are killed, Bling! contemplated leaving the school and Onyxx intended to follow her.  However, the two remained at the Xavier Institute until it was destroyed and all students were sent home.  Because he was an orphan, it is unknown where he had went following the closing of the school.

Utopia

Onyxx returned under the X-Men's wing when they relocated to San Francisco. When Simon Trask incited an anti-mutant riot over the creation of Proposition X, Onyxx was seen with the X-Men trying to make order of the chaos and protect innocent civilians caught in the crossfire. He and Ariel arrive in helping Gambit rescue Trance, a student who was lost in the riots.
 

Necrosha

Onyxx was among the X-Men on site when Selene's Inner Circle attacked the X-Men.
 
When former X-Student Wither tried to kill Dust with his matter decaying power, Onyxx leapt towards Wither to stop him.  Wither quickly turned around and used his death touch to disintegrate Onyxx's body into dust, only leaving behind his helmet which Dust mourned over in agony.
 
 

Second Coming

 By unknown means, Onyxx reemerged during the Second Coming crossover and was present during Bastion's attack on Utopia.This is possibly a mistake.

Powers and abilities

Onyxx is a mutant whose body is composed of rock, granting him enhanced strength and durability. He also has four eyes and has a metal sphere encasing his head.