Koume is a 3D model original character of a young fox girl child made for VRChat by Hamuketsu.
She normally wears a brown pinafore dress over a blouse with puffy long sleeves decorated with a string tied in a bow, both ends terminating into flowers (plum blossoms?). She also has short blonde hair with another string hair ornament, tying a one side up and decorated with more flowers.
There are viral videos of her playing Beat Saber, which is why she is sometimes associated with that game.