From here:

Mitsu is in love with Madoka, who doesn't even know she exists. Mitsu's pretty happy stalking Madoka, but is a little careless about it and, one day, she's hit by a truck and killed. But now she can REALLY stalk Madoka - and drive away all the boys - and girls - that show any special interest in her. Woo-hoo! Plus she gets to be there when Madoka changes clothes and bathes, etc. etc. Mitsu's pretty happy with the deal. Then transfer student Kyouko shows up, declaring that when they were kids, Madoka and she had promised to marry one another. Mitsu tries her best to scare Kyouko off, but Kyouko is actually able to see Mitsu and isn't about to be budged.