Tsundere

Meaning of the term “tsundere”
The Japanese term tsundere refers to an outwardly violent character who "runs hot and cold", alternating between two distinct moods: tsuntsun (“aloof or irritable”) and deredere (“lovestruck”, “affectionate”).

The term was originally used to describe characters who are initially cold, or even hostile (often towards a love interest or people they care about) before gradually warming up and becoming sweeter over time. This type of character is known as a Classic Tsundere (examples are Rin Tohsaka, Lum from Urusei Yatsura, and Misty from Pokémon).

The definition of “tsundere“ has changed over time. Now, it is generally used to describe a character who rapidly switches between tsuntsun and deredere instead of starting out tsuntsun and slowly becoming deredere. These characters usually snap at the smallest provocation, and usually at a specific person rather than a general sociability problem, and show little to no true character development. This type of tsundere is often referred to as “Modern Tsundere”. Examples of a character with this personality are Sistine Fibel (Rokuaka) and Meidri (Ishuzoku Reviewers).

Overview
The tsuntsun can range from the cold "silent treatment" to the hotheaded "kindergartener who pushes you into the sandbox." The reasons behind a Tsundere's behavior vary widely, but usually boil down to the conflict between their feelings of affection towards a love interest, and their reaction to having those feelings.

The Tsundere stock characterization is very popular with writers of Romantic Comedy because the conflicts between the two personality facets can be easily utilized to generate both drama and comedy. It also acts as a source of Wish Fulfillment: specifically, the idea that every independent, hardened and just plain jerkish love interest (male or female) has a squishy emotional center that will embrace you after you crack the outer shell.

Tsundere can be divided into two main categories, depending on their default mood: 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 Harsh to Sweet. If her motivations are inquired, she will often engage in a Suspiciously Specific Denial, complete with a Luminescent Blush and total evasion of eye contact (cue the squeaks of Moe).
 * Harsh (or Tsun): These Tsundere have tsuntsun as their default mood. It takes someone special to trigger their deredere side. The intensity of the tsuntsun can range from simple grumpy pessimism to "I must glare and fight my way through life". It's about which part of the tsundere personality is the public face and which the hidden. If the Tsundere is The Rival, she is more likely to be Harsh. Helping a rival out is usually accompanied by a line like "Don't get me wrong, I'm not doing this for you."  Harsh types can overlap with a Jerk with a Heart of Gold, but usually not. The moods of a Tsundere tend to switch in reaction to the actions of select people or adverse scenarios; the deredere side usually only comes out when someone has acted in a way to trigger it. A Jerk with a Heart of Gold is jerkish in general regardless of whether the other person is mean or nice, and shows their Hidden Heart of Goldonly when the situation warrants, regardless of how the other person had been acting. Male characters in particular should be considered for Jerk with a Heart of Gold status, as arguably because of Double Standards, men are generally that instead of tsundere, although the kuudere subtype is more equally split in gender. Oranyan is sometimes used to refer to a male tsundere character—incorrectly since it means the complete opposite.
 * Sweet (or Dere): 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. Don't confuse the sweet tsundere with Bitch in Sheep's Clothing because in this case, they have a temper almost always triggered by someone or something else, usually a Love Interest. Either they have Belligerent Sexual Tension, are an Accidental Pervert, or just have no idea how to handle feelings of love and attraction. In some cases, an Armoured Closet Gay character may act like a Tsundere to mask their feelings for the object of their same-sex affection. May also overlap with Violently Protective Girlfriend if her Love Interest is threatened or in danger.  Sweet types should also not be confused with a Yandere. If a Sweet Tsundere were really convinced that their Love Interest didn't want them, they would revert back to the deredere side and probably enter an I Want My Beloved to Be Happy phase, while Yanderes are Not Good with Rejection at all and have been known to get downright murderous under such circumstances.

When paired with a Jerk with a Heart of Gold, together they produce Belligerent Sexual Tension. If done poorly, the result is an Unintentionally Unsympathetic Jerk Sue. Compare with Well, Excuse Me, Princess!, Jerkass, and Jerk with a Heart of Gold. Contrast with Sour Outside, Sad Inside, which shares the spiky exterior but has depression and self-doubt rather than kindness hiding underneath. When Flanderizedtends to overlap with Mood-Swinger. Also see Don't You Dare Pity Me! and Anger Born of Worry; both of them likely actions with this character type. The more benevolent examples are very likely to be Good Is Not Nice (or anti-heroes). Aloof Ally may show the same hot-and-cold behavior but for differing reasons. Shana Clone is a specific subtrope with a particular set of characteristics. Because of their low tolerance for stupidity, they are always Enraged by Idiocy. These character types can fit both variants of Jerkass to One(for the first variant, they only act harsh with their love interest while is gentle with everyone else, and for the second variant, they act harsher with their love interest more than they are with others.)

There are a few male examples, but the tsundere is more likely to be female and interested in a male protagonist. This is likely due to the Unfortunate Implications of a male love interest constantly deriding and insulting a heroine because of his unspoken feelings for her. Male love interests with tsundere-like traits are more likely to have mutual Belligerent Sexual Tension with a girl equally as tsundere as he is, or act aloof and detached as more of a Kuudere type character.

Psychologically, tsundere-like behavior could be an example of "splitting", a maladaptive coping mechanism wherein a person alternately idealizes and undervalues others, including potential romantic partners.