(NOTE: THIS KIND OF PERSON CAN BE OF BOTH GENDERS)
The Japanese term tsundere refers to a character who "runs hot and cold", alternating between two distinct moods Known as: "TsunTsun" (Aloof) and "DereDere" (Lovestruck)
The tsuntsun can range from the "silent treatments" to "That lovestruck kindergartener who pushes you into the sandbox." The reasons behind a Tsundere's behavior vary widely. Sometimes there are reasonable justifications for her actions. Other Times there are simply flow from the conflict between their feelings about the object of their affections and their reactions to having them.
The Tsundere is very popular with writers of Romantic Comedy because the conflicts between both personality facets can be easily utilized to generate both drama and comedy.
There Are two types of this character:
Type 1/harsh: These Tsundere have tsuntsun as their default mood. It takes something special to trigger their deredere side. The intensity of the tsuntsun behavior can range from "I must glare and fight my way through life" to grumpy pessimism. It's about which part of the tsundere personality is the public face and which the hidden. If the Tsundere is The Rival to the main character, she is more likely to be Type 1. Helping a rival out is usually accompanied by a line like "Don't get me wrong, I'm not doing this for you."
Type 2/Sweet: These Tsundere have deredere as their default mood. They are sweet, kind and generous, but just happen to have a hidden violent side as well. In this case, temper is almost always triggered by someone or something else, usually their Love Interest. Often if they are insulted in someway.
Eaither way both have one thing in common.......they're very fickle women, no offense to anyone who is reading this.
A common way of showing that a Tsundere has mellowed or has had her heart won over by the Love Interest is to have her shift from Type A to Type B. If her motivations are inquired, she will often engage in a denial of their own feelings.