Originally used by Nana Komatsu, this is a name she made up when she kept imagining that her boyfriend Shouji was having a secret relationship in the anime/manga "Nana". Eventually, he did cheat on her, with a girl named Sachiko Kawamura.
A Sachiko is often described as a "man stealer".
A Sachiko is actually really sweet and is unaware of the man she's dating's other girlfriend/wife
Shouji and Sachiko go to the same school, are going for the same major, and even work together! Kyousuke, who only likes tan girls even thinks she's cute. This could be bad... And it did get bad. Go watch Nana episode 14.