Best definition
Yan-chan is another word for Yandere, but to make it a nick name for someone, or what to call a Yandere. A Yandere is someone who usually pretends to be sweet, but in reality they are someone who will kill for a certain reason. That reason is: when a Yandere finds a crush (or senpai) they will kill anyone who will try to harm their crush or anyone else who likes them.
Person#1: wow, we should call her yan-chan

Person#2: how so?

Person#1: Well she’s a Yandere

Person #2: why?

Person#1:Because she threatened to kill a girl that had the same crush as her!

via giphy