A Fourth Party, Rin
The biggest change being implemented in Catherine: Full Body is the inclusion of Rin, a new character and potential third love interest for Vincent.
Introduced as an amnesiac girl who Vincent rescues in a dark alleyway, Rin takes up work at the Stray Sheep as a pianist while trying to recover her lost memories.
For unknown reasons, she also makes an appearance in the nightmare world as a pianist, helping to slow the fall of the blocks and giving the sheep more time to climb to safety.
There’s a whole new story route to pursue with Rin in Catherine: Full Body, allowing Vincent to turn away from both Catherine and Katherine, and go for a completely different path instead.