A colleague told me, "This flower bloomed in the garden."
It is hotarubukuro (Campanula punctata), bought as a potted plant 20 years ago and now multiplied in the garden. This stem is about 30 cm; they can reach 70–80 cm, so it may still be growing. The flowers are fairly large at about 7 cm and should be enjoyed for another month. (The slightly deeper purple flower toward the back right in the photo is shiran—Chinese ground orchid.)
Hotarubukuro is a perennial in the bellflower family, native to East Asia including Japan. A mountain variety, yamahotarubukuro, looks much the same, white flowers included, but hotarubukuro has reflexed appendages between the sepals, while yamahotarubukuro's sepals bulge between them.
Written "firefly bag" in kanji, the name likely comes from the idea that light from a firefly sealed inside would shine through the flower. Another theory derives it from an old word for lantern, "hotaru bag," and it is also called chochinbana (lantern flower).
Similar is tsurigane-so (bellflower), another member of the hotarubukuro genus. Tsurigane-so has many species with varied colors and shapes, but hotarubukuro flowers tend to look more elongated. Some call hotarubukuro tsurigane-so, but tsurigane-so properly means campanula in Latin—bell in English—from the Mediterranean.
In our April 10 blog we noted Genji fireflies near Ushigafuchi by Palaceside Building (between Kudan Kaikan and Kitanomaru Park). Putting fireflies in flowers—more romantic than any literal "fishing bell," I love that name~.(^―^)