On other occasions I have spoken with hidden nooks for storing clothes, for storage, drawers, etc. Sometimes people choose to fill this gap of shelves and manufactured its own library.
Many times, these places are ideal for creating small rooms as a study, with a desk that can hide in that little hole.
If the house allows, it must first ensure that the downspouts match this corner, is a very good option. A tucked under the stairs where we can freshen up before leaving home.
As for the decoration of this space, being so small, it is best to cover the walls, even furniture with light as possible, ie, with colors like white. Although this is ideal, it is certain that if the staircase is wide, can grant a license to use some other pattern. Although as shown in the following image, there are tastes for everyone.
Finally, we recommend the option of incorporating some element in wood , and it is this type of hidden corners much better off with wood. Les brings a touch quite rustic and even romantic.