Any way Rie was so happy to see Mari again that she couldn’t hold it in anymore so she of course kissed Mari to whom it might have seemed right out of the blue. But Rie was not kissing Mari, well she was but she wasn’t and let me clear this mess up before anyone gets lost. As mentioned Rie is a ghost, which means she has lived for a long time seeing as even the school uniforms have changed. In fact when she was alive, Satsuki’s grandmother was their classmate (which goes to show how important that grandma is). We learn that back in the day, even when yuri was most un-heard of, there were two girls who were not afraid to let their feelings for one another known despite the negativity they must have received especially in such a rural area. They were mentioned to have been so happy and that many were more envious of their happiness than anything else but of course as we know the world doesn’t work like that.