I feel like you'd need a character to develop in order to have character development.
A tutorial, 3 probes, an email, and 2 cameos in sex scenes has shown very little of the Rival beyond "I'm you spoilt cousin who may or may not be racist and also kinda a bitch but your dad said not to hold it against me despite the fact that I never give you much of a reason to care for me outside of a bad end." And that is me being generous.