Character(s) located on a rooftop. Rooftops are roofs purpose-built to be walked on, a feature common in many Japanese schools, certain styles of apartment blocks, buildings with pools, gardens, lounges atop them, etc.
Do not use on rooftop with on roof. On_roof should only be used together with roof, the tag for roofs not intended for human traversal, like those present on traditional homes or pagodas. (Danbooru post #5891954)
