White Knight
A person (usually a male) who sees the typical maiden in distress, and believes that he can help her. A male version of the "mother figure" that some girls become.

White knight term can also be applied to a person participating in online forum who is not tasked with moderating it, but engages in the activity nonetheless - so-called backseat modding. The underlying causes for the behaviour are the same - person is anthropomorphising the forum and sees it as a "maiden in distress" and charges to the rescue.
"Why is he going out with her? She's broken, and a little crazy."

"The fool's just being a White Knight."