San Ichi Tan, commonly known as 3.1-tan, is a personification of the Microsoft Windows 3.1 operating system from the OS-tan series. She is depicted as a young, energetic girl wearing vintage computer-themed attire, reflecting the era of her namesake OS. In the OS-tan universe, 3.1-tan plays the role of an elder sister to later Windows versions, often showing a nostalgic and slightly old-fashioned personality. Her character embodies the transition from early graphical user interfaces to more modern systems.