Alexander Summers or Alex Summer to friends, is the baby brother of Scott Summers a.k.a Cyclops. Both are survivors of an alien abduction, this abduction left them orphans. He was placed in an orphanage in Omaha run by Mister Sinister. Alex was adopted by a mourning family unlike Cyclops who joined the X-Men.
Sinister, helped trigger Alex's mutation by persuading a school bully to attack Alex. This caused Alex's mutant powers to manifest and he incinerated the Bully. Sinister, then put a genetic lock in Alex's mutant gene, this stopped the development of his mutant powers. Alex did not realized he was a mutant until after graduating college.
Alex, went to Egypt and there, he ran into trouble with who we now know as the Living Monolith. There he was the subject of experimentation and involuntarily help create this villain. Thanks to the X-Men, Alex, escaped and later was captured by a Sentinel. The inventor of the Sentinels gave Alex a black costume that helped control his mutant powers and he also gave him the codename: Havok.
Again the X-Men helped Havok escape but unfortunately he ran into another villain Karl Lykos, whose power consisted on feeding off other mutants. When he fed on Havok, he transformed into the villain we know as Sauron.
After that Alex, joined his brother in the X-Men. He fell in love with teammate Lorna Dane, better known as Polaris and after a brief time with the team they decided to retire and have a normal life. But this was never meant to be as the Marauders began attacking them. Because of that Lorna became possessed by the entity known as Malice, who forced her to attack the X-Men as the Marauders’ new leader. Alex had no choice but to fight her, with help from the X-Men.
Havok spent some time in Genosha away from the X-Men again. Eventually accepted an offer to lead the second group of X-Factor, a government-funded mutant agency, of which Lorna was a member as well.
Alex continued leading X-Factor, until death of his teammate Jamie Madrox, the Multiple Man. Depressed, Alex joined the Dark Beast (The Beast from Age Of Apocalypse) as a member of the Brotherhood of Mutants. However, Havok, was there to stop the Dark Beast's evil plans. He did and joined another team the Xavier Underground Enforcers (they were a big help in Alex's plans to stop the Dark Beast) this time with the team, his mission was to stop their apocalyptic future from occurring. Alex was killed when a time machine exploded, but he was actually shunted into an alternate reality, trading bodies with the Havok of that reality (in that reality Havok was evil) However, everybody in the Marvel universe thought he was dead.
Alex again died, but this time his conscience was traped in Limbo. Back on the Marvel U. His comatose body appeared in a hospital and was identified as Alex Summers. His nurse Annie Ghazikanian, had fallen in love with Alex and with her help and her son's help they were able to bring him out of the coma. But things got complicated when he accidentally got engage with Polaris, Alex left her at the altar, when he realized that he was in love with Annie. Polaris went all kinds of crazy and attempted to kill both Alex and his lovely nurse, but thankfully she was stopped by the Juggernaut.
After that the bad alternate reality Havok started awaking within Alex's consciousness and started causing tons of trouble for Alex, lucky for him the dimension-hopping group known as the Exiles were after the evil Havok and they teamed up with the X-Men to stop him. They did and saved Alex in the process.
Recently, after the events of House Of M. A lot of mutants lost their powers, including Polaris. She left the X-Men and Havok decided to go with her. Later Polaris got abducted and Havok much like Luke Skywalker said "there's nothing for me here" and returned to the X-Men and the Starjammers.
Wow, this guy's life is complicated but what you really need to know is that he is Cyclops' baby brother and he has the power to explode things with the bursting energy he shoots from his hands or if you want a more technical explanation: he is able to absorb ambient cosmic energy, his body is able to transmute this energy and then release it as waves of energy that when they hit the air they turn to super hot plasma.
His first appearance was in Uncanny X-Men # 54. Best runs featuring Havok are all his X-Factor (from issue 71-149) and the Mutant X series. You can check his X-Men run too but is not as memorable. Havok, is a cool character with a horrible backstory that for some reason things keep happening to him twice. He died two times, was responsible for the creation of two villains and retired from adventuring, with Polaris, two times, so wait maybe this could be a second mutation... Hey maybe I found something here-- hey Marvel do I get a No-Prize? hey... Oh well...until Havok rejoins X-Factor for the second time, go to the movies and watch X-Men: First Class.