She is Tsukunes mother and used to prepare snacks for the kids when Kii came over to play with Tsukune.
Kotone is part of the 7th generation of mahou shoujo, her rapid healing powers manifesting after she gave birth to Tsukune.
In 2030, she supplies Seiichi with a test tube of her blood, which he passes on to Sou Shirokane in hopes of aiding in the resurrection of his wife.