Mark

A caring individual that will continue to keep giving to you and caring about you even after you have royally screwed them over or treat them like shit. This is mainly due to low self esteem and/or a bad self image brought on by a traumatic childhood and or abusive/poor parenting skills.
The mark usually will continue to stay in abusive relationships where they are constantly taken advantage of and cheated on due to their craving for the approval of the abusing partner and believes that if they do/give more to the abusing partner they might achieve this. In all actuality it will never happen for poor ol' mark and will only get worse as time progresses. The mark stays with someone for fear that they can and will never be able to find someone that truly loves them for who they are.

Hey alicia, that guy looks like an easy mark. You could be set for life with him!