Mac OS5
Macintosh System 5-tan actually represents Macintosh System Software 5 (System 4.2 + Finder 6.0) and Macintosh System 5 although the latter does not actually exist. System Software 5 introduced Multifinder (allowed for multitasking), a color interface and labels.
This more common appearance as Mac System 5-tan is a phantom, invisible to those outside the Mac family and her existence is believed to only be an urban legend by outsiders.
She has dark purple hair in a ponytail, topped with a leaf-shaped cowlick, is considerably taller than her older sisters with a height of 5'3 [160 cm], has a pink left eye and silver right eye, wears a Phantom of the Opera-style mask, frilly purple+white dress, black gloves, cloak and white boots and is a talented magician, fortune-teller, organ player and prankster who especially likes pranking outsiders. Her closest friends are Mac System 4-tan and Mac System 6-tan as they share some of her personality traits.
Her human alter-ego is Mac System 4.2-tan.