This is not Rimururu, so I will change the tag. this is definitely the Bust version of Nakoruru in Samurai Spirits 64 or Warriors Rage. Her true name is Rera.

But this one has Rimururu's brown hair and hair style. It looks more like Rimururu cosplaying as Bust Nakoruru.

Here is the official web site. http://samuraianiv.snkplaymore.co.jp/english/character/nakoruru/index.php There are mini pics of Nakoruru in each series and this version of bust Nakoruru (THe picture shown) is in the Samurai Spirits Warriors Rage section in the series. http://www.nakofan64.net/nako64.html This also show the entire pic in the bottom right of the page which shows Bust Nakoruru.

How beautiful.

What exactly was the difference between the "Slash" and "Bust" versions of the characters in the Samurai Spirits games anyway?

Mainly, there'd be a difference in movesets. Sometimes the characters themselves would take on a "dark" personality of sorts in Bust mode, especially in the later games.