Fan nickname given to the doll that accompanies Medicine Melancholy from Touhou. Originates from what Medicine calls suzuran (lily of the valley) in PoFV. The doll doesn't actually have a name canonically.
Her official artwork from PoFV depicts her having short blonde hair with a red ribbon, blue eyes, pinkish-white wings, and an outfit with a red top and black skirt (the inverse of Medicine's outfit colors.) However, like with the other dolls in the series, her appearance often varies by artist.
Given Medicine is a tsukumogami, it's speculated that Su-san is actually the real Medicine and contains her soul, the larger main body being a projection, similar to the speculation of Tatara Kogasa's nature as a karakasa obake.