So, basically, you have to do Kokichi's freetime event stuff.
On his last one, after they do the knife thing ( that's Also another reason why), Kokichi says " I stole your heart, so now I'm satisfied "
This one is a bit far stretched..
Um, in Kokichi's love hotel, Shuichi is still a detective. I am ignoring the unholy stuff kokichi says, but that is evidence