San-ichi Tan, also known as 3.1-tan, is a moe anthropomorphism of the Windows 3.1 operating system in the OS-tan meme series. Portrayed with a retro look, she often wears clothing reminiscent of early 90s fashion and is depicted as reliable but somewhat outdated compared to her successors. Her role in the series is to embody both the strengths and limitations of Windows 3.1, often serving as a supportive, nostalgic figure to other OS-tans while referencing her legacy in computer history.