Tsundere is a Japanese word where a character is cold or hostile at the initial part of a story that slowly becoming friendlier and starting to have a warm personality. Tsun means to turn away and dere (dere) means being sweet. It is believed that the first tsundere is Lum Invader from Urusei Yatsura manga series. It ran from 1978 to 1993 and also had an anime adaptation.
Lum is a royal and daughter of an alien invader to the planet Earth. She is much of a laid-back character but is very quick to be angry. But she is all loving to Ataru who she is in love with. Lum embodies a modern Japanese woman who is independent yet a supportive wife and mother.
According to several sources the first tsundere character is Lum Invader from Urusei Yatsura and is rumored to be the basis for all moe. Moe is a Japanese term which is slang for intense feelings for different major characters in anime, manga, and video games. This term has now evolved to stnd for any affection towards any character or situation.
Urusei Yatsura began selling manga in 1978 and Lum had a very cold and standoffish dimeanor while being warm and loving on the inside. These character traits earned her the title of the first tsundere.