The Scarlet Witch
Real name: Wanda Maximoff (or Wanda Frank, depending on when you started reading)
First appearance: The Uncanny X-men #4
Favorite quote: "Flee, whoever you are!" (Avengers 76; the first words she ever spoke to the Vision, her future husband)
Team affiliation: Long time member of the Avengers, including a stint with the now defunct West Coast Avengers. Founding member of Force Works, also defunct, and the Brotherhood of Evil Mutants, which crops up with new members every now and again. Her most important partnerships, however, are with her family. Wanda's twin brother is the fleet (and foul-tempered) Quicksilver, while her father is the dangerous and powerful Master of Magnetism, Magneto. She was married for many years to the syntheziod Vision, until both he and their children were destroyed in the single most lame storyline Marvel ever committed to ink and paper...but that's another story...
Powers: Wanda has the mutant ability to alter probabilities, as well as a fair amount of skill in the mystical arts.
Favorite storyline: the long-running romance between her and the Vision, culminating in their marriage.
Least favorite storyline: The revelation that her twin sons were figments of her imagination. Brutal, and the start of a long downward spiral for the character from which Wanda, once one of the leading heroines in Marvel comics, has never recovered.
What's in a name: Apparently some people are now calling the Scarlet Witch Wanda Lensherr, after her acknowledged biological father Erik Magnus Lensherr, AKA Magneto. Frank is from her once presumed biological father, the Whizzer, and Maximoff is from her adopted father. Has anyone thought to ask Wanda which name she prefers? In the limited series starring herself and her then-husband the Vision, she's on record as not wanting any last name at all!
Family Portrait: Wanda and her brother, Quicksilver, appeared with Magneto many times, long before they knew the Master of Magnetism was their father. Look below for a great early Avengers cover featuring the three of them.