Main character design
A rarer character, MS-DOS is a cute but very shy little schoolgirl carrying around a keyboard and usually peeking around corners. The implication is that, like MS-DOS itself on non-NT versions of Windows, she is always lurking somewhere in the background, just out of sight.
She originally represented the DOS prompt (In early pictures, including the one in the infobox, her armband says "Prompt") and assumed to be a catch-all for all the DOS OSes, but now more specifically represents MS-DOS.
Other character design
Another representation of MS-DOS is Windows 3.1-tan's cat, who can shapeshift into a catgirl in a nurse's outfit. It isn't known if she was an alternative MS-DOS character (since the 'main' MS-DOS-tan is rarely drawn), but in most fanon, both characters co-exist.
History and background (fanon)
In the 1980's, she happily grew up as part of the Microsoft-IBM Family and lived with PC-DOS-tan, Windows 1.0-tan, Windows 2.0-tan, their stepsister OS/2-tan, and Xenix-tan the family caretaker. MS-DOS-tan and PC-DOS-tan were especially close and had nearly identical personalities but the break-up of the Microsoft-IBM Family changed both of their personalities radically, and during the 1990s, they were only able to confront each other as rivals.
MS-DOS-tan lives with the present day Windows Family faction, but is a timid shadow of her former self, not just from the Microsoft-IBM Family break-up in 1990, but also from her diminishing popularity and influence.
Although a pacifist during the OS Wars, she avoided the fate that Windows 1.0 and 2.0 suffered due to passively complying with the Family's imperialist ambitions, and because she was still influential during that time. It was by the time that ME-tan was born that her influenced diminished radically (ME cannot boot DOS without a bootdisk, and is the last DOS-based Windows). She cares for all of her family, but can't seem to relate to them anymore except for 3.1 and 95 in the Windows Family; 1.0, 2.0 and PC-DOS in the Binteeji Renmei.
She and PC-DOS-tan have the habit of reuniting (in secrecy from the Windows-tans of the Windows Family, who do not know of PC-DOS-tan's existence) with each other every so often and when they do, both of their personalities change dramatically, to their cheerful former selves.