Zane is an illegitimate son of Straff Venture, and therefore half brother to Elend Venture. Zane is one of many half-skaa who were fathered by Straff because of his desire to have a Mistborn under his control. Thanks to his harsh upbringing and Hemalurgic spike which let him hear the destructive suggestions of Ruin, he is an unstable person.
Zane is the Mistborn Straff Venture wanted and therefore is one of the most deadly characters in the series. Thanks to his spike, his ability to push on metals was enhanced. This lent him extremely great precision. 
Thanks to his Hemalurgic spike, generally harsh lifestyle, and lack of meaningful human companionship, Zane is a rather unstable individual. He thinks he is crazy because of the psychotic voice in his head, not knowing about the nature of Hemalurgy. He had some feelings for Vin because she was the only one the voice in his head would not command him to kill. He eventually asked her to abandon her current love Elend and be with him. When she decided to remain with Elend, he attacked with deadly force shouting she was supposed to help him regain his sanity. When she managed to defeat him, just before his death Ruin let him know he was never really insane.