Rokk Krinn was one of the biggest magnoball stars on his homeworld of Braal, known as "Cosmic Boy" to his fans. The sole support of his family, he was unaware that Alex Cuspin, his manager, was embezzling most of his earnings.
One day, he was going to Earth in an attempt to earn extra money. The trip would change his life forever...
On the shuttle, he met Garth Ranzz, and the two immediately hit it off, but they would receive a frostier reception from Imra Ardeen, a blonde telepath and Science Police cadet who caught Garth mentally undressing her. On landing, though, Imra mentally "spotted" three assassins going after R.J. Brande, who Rokk and Garth made short work of when she directed them to stop them.
They then went their separate ways, but Rokk, tired of Cuspin's bullying, went with Triad-Purple when asked to a meeting with Brande, where he found that Garth and Imra had also been summoned - whereupon Imra promptly revealed how Cuspin had been embezzling from Rokk and he immediately fired him.
After being invited into his office, having been shocked and delighted by their performance, Brande offered them the chance to become the founders of the Legion of Super-Heroes. They accepted.
Shortly thereafter, Brande would arrange for Rokk's parents to get jobs in one of his Braalian factories.
Rokk, taking his old nickname of "Cosmic Boy" as a codename, proved to be a natural leader, helping to smooth over the early tensions between Imra and Garth. When Leviathan was drafted as leader and the first Kid Quantum was killed, he recognised his shortcomings and quickly turned leadership back to "Cos", as he became known.
Over time, however, he would appear to become more irascible and by-the-book, following President Chu's orders without question. This was, in fact, an act designed to let him get close to Chu while instructing Invisible Kid to form an "Espionage Squad" to investigate Chu, and Live Wire (Garth) to form a "Rescue Squad" to bail them out at the appropriate moment. It would work, and Chu was indicted, while Brande was drafted as U.P. President, and his first act was to give the Legion autonomy over their own membership.
Immediately after, Cos would stand down as leader, feeling he'd served his time in the role.
However, Shrinking Violet, the new leader, would prove to be under the control of the Emerald Eye, and send half the Legion, including Cos, back in time. There, he would instinctively take over the leadership, while entering into a previously-teased relationship with Saturn Girl (Imra). However, he would receive a head injury, leaving him comatose for some time. He apparently recovered, but in fact Saturn Girl, the limits on her powers accidentally removed by Doctor Psycho, was subconsciously puppeteering him - a fact only revealed when the two almost married, and Irma's mind rebelled against the idea of marrying someone other than Live Wire. Nonetheless, although that was the end of them as a couple, they remained friends.
After returning home, Cos would have to deal with his former manager once more, who, now released from prison, hired an assassin calling himself Domain to kill Cos, but he - with some help from his brother Pol - managed to survive, and Domain would prefer suicide to capture.
After half the team was Lost in a Rift, and the team formally disbanded by President McCauley, Cos was left at a low ebb, but Triad and Brande would quickly recruit him once more, as head of the "Legion Subterfuge Squad" with Violet, Invisible Kid and Chuck Taine, while Brande quietly constructed Legion World in secret.
As part of this, he had himself deliberately arrested for vigilantism on Braal as part of a plot to steal a Footstep Drive ship, the only means of rapid transport between planets with the Stargate network shut down. As this came to fruition, however, bringing him and his team into conflict with Magno, the Lost Legionnaires returned, and their first trip was to Earth to find their teammates...
Meeting up with Triad, who had rescued Shikari from imminent vivisection, Shikari guided them to their teammates, finding them on the run in a toxic waste pipe, where they had also found the remains of the real McCauley.
He instantly fell into his old leadership role, and after a trip to Legion World to let their teammates recover for a few days, while Invisible Kid and Gear helped Brainiac 5 to develop the "Threshold" that had got the Lost team home into a viable transport system - albeit one dependent on the Kwai's help to pilot the teleportaion-based system, requiring a team dispatched to engage their help.
This done, the newly-reformed Legion held a leadership election. Thanks to Cos' own quiet campaigning on her behalf, having observed her during the fight against Ra's, Jazmin Cullen, the second Kid Quantum was elected - much to her shock. The two would later enter into a relationship.
Powers and abilities
Cosmic Boy could manipulate magnetic forces to levitate ferromagentic objects or people wearing sufficient quantities of ferromagnetic substances, fly, create forcefields and reshape sufficiently soft ferromagnetic objects, such as the four disks he wears on his chest.