Kana has feelings for Riku, seeing how he cares for Ayana despite knowing how she made his life terrible by accidentally killing his parents and sister.
Dunno why she needs his care, she seems like a person without issues, unless there's some dark past she is hiding...
Basically, seeing how Riku is hurt by Ayana accidentally killing his family and Ayana blaming herself, decided to go forward with her feelings to replace Ayana for herself to heal the heart of Riku.
Ayana now didn't get back to her delusional world, blaming herself for the accident, not thinking her suicide would actually hurt Riku more than what she has accidentally done. Can't really blame her in her current state of mind, but she doesn't think clearly.
Also, what's up with Chiharu ... I know she does doujinshi and is a landlady but...