Havok
Real Name: Alex Summers
First Appearance: Uncanny X-Men 58
Mutant Abilities: Generates cosmic radiotion blasts
Details: Havok joined the original X-Men team but didn't stay for long. He proceeded to join and leave several different teams, constantly worrying about his powers. Finally, he was captured by Random who brainwashed him and boosted his powers. He is still on the bad guy's side
Mutant Relatives: Scott Summers (brother), Corsair (father), Jean Grey (sister-in-law) and the many Scott Summers offsprings from various realities
Relationships: Polaris
My Opinion: Contant whining and worrying made him incredibly unlikable. His relationship with Polaris was interesting and it was gratifying to see him betray her time and time again

Magneto
Real Name: Erik Magnus Lensherr
First Appearance: Uncanny X-Men 1
Mutant Abilities: Control over magnetism
Details: Forced to watch his family die during the Holocaust, Magneto then journeyed to the jungle where he met Charles Xavier. The pair worked together for a time, before their conflicting views about the future of mutant-kind forced them apart. Magneto became the X-Men's main nemesis until, oddly, he had a change of heart. He even replaced the Professor for a short time. Unfortunately, this didn't last long and Magneto returned to his old ways, forming a mutant team known as the Acolytes. He dedicated himself to creating a mutant haven. He then became part of the entity known as Onslaught while his clone, Joseph roamed the world, believed to be Magneto by many
Mutant Relatives: Quicksilver (son), Scarlet Witch (daughter), Joseph (clone)
Relationships: Rogue, Magda
My Opinion: You gotta love Magneto. He has Mutant-kinds best intentions at heart but he always seems to go about everything in the wrong way. If I didn't love Gambit so much, I'd say Rogue should have stayed with Magneto!

Mimic
Real Name: Calvin Rankin
First Appearance: Uncanny X-Men 19
Mutant Abilities: Can 'mimic' any living creature
Details: A member of the X-Men, Mimic left to join the Brotherhood of Evil Mutants. His powers are now limited to the five original X-Men
Mutant Relatives: None
Relationships: None