Lived long enough to become the villain (Koromaru, cuz im SOOOOO evil waifu. ;D)
There was little love left in the white dog. After losing Beanie-kun and then the quiet one, Koromau couldn't be around the others anymore. He'd lost his previous owner too, and felt a deep connection with the other two that passed on.
So rather then try to find comfort in the arms of another owner, he found himself attacking anything that he could. His fur was no longer white, but a dark crimson. A mixture from the other dogs he's killed to keep his territory and from the humans he bit to be left alone.
It was a terrifying change so he became more feral in order to prevent the fear from gripping him. The fear that could very well mean that he wouldn't be able to use the Persona that he still needed to protect himself from Shadows.
Late at night he would curl up and dream of Beanie-kun. His heart was blocked off from everything, but the memories of his friend was the only thing that got through that armor.
