Tsundere (ツンデレ, pronounced tsɯndeɾe) is a Japanese term for a character development process that describes a person who is initially cold (and sometimes even hostile) before gradually showing a warmer, friendlier side over time. The word is derived from the terms tsun tsun (ツンツン), meaning to turn away in disgust, and dere dere (デレデレ) meaning to become 'lovey dovey'.Originally found in Japanese bishōjo games, the word is now part of the otaku moe phenomenon, reaching into other media such as maid cafés, anime, manga, novels, and even mass media. The term was made popular in the visual novel Kimi ga Nozomu Eien.

Noun: a friend who is usually only called upon by a friend when a more primary/prominent friend is unavailable.

Taken from the term “second string” in an athletic competition situation. In football, if the star quarterback gets injured during a play, the second string is called off the bench to replace him/her. A “Second String Friend” is essentially benched until needed, if ever.

Synonyms: Plan B, Second Choice, Secondary Friend, Benched Until Needed

Tsundere is a Japanese term for a character development process that describes a person who is initially cold (and sometimes even hostile) before gradually showing a warmer, friendlier side over time. The word is derived from the terms tsun tsun, meaning to turn away in disgust, and dere dere meaning to become 'lovey dovey'.

A girl who hates the fact that she loves the fact that she's in love with you

As you've heard already, this is a anime stereotype where a character is violent and defensive at first, but sweet and affectionate in later on.

Tsundere is a slang born on the Internet, and it is a word to describe the nature of female anime or game characters. Tsundere is a word combining two words, "Tsun Tsun" and "Dere Dere". Both terms describe attitudes of a person. "Tsun Tsun" is used for cold/blunt/curt attitude, while "Dere Dere" is used when a person becomes spoony in front of his/her lover.
(Definition is from the game " Tsuyokisu Cool×Sweet ")

1: being on the surface sharp and sarcastic (tsuntsun) but underneath lovestruck and fawning (deredere); characteristic of a gap between acted out actions and feelings in mind;
2: normally being sharp but at some prompt suddenly becoming lovestruck; hot-cold personality type