Lain's sister. At the beginning of the anime, she is a regular teenage girl who often stays out late with friends -- a stark contrast to her utterly asocial and lonely sister.
In the second layer, Mika is curious of Lain's newfangled interest in NAVIs and the Wired, at first commenting on Lain's vocal interactions with her NAVI as talking to her 'imaginary friend'. She reminds her sister that she will be late for school, and later watches her father set up Lain's new NAVI, but feigns disinterest when he notices her eavesdropping.
Mika encounters the Men in Black at her door in the third episode, and threatens to call the police; they simply respond that she never saw them, nor were they ever here. She later tells her mother to call the police if the shady pair ever return. Mika visits Lain's room, only to find her sister tinkering very adeptly with computer hardware and wearing nothing but her underwear. Lain says that she heard that she had to look out for static electricity, so she decided to take her clothes off. At Mika's bewildered response, Lain happily welcomes her sister home.
In Layer 04, Mika only comments that Lain is "weird", perhaps due to her confusing shifts in personality and demeanor, as well as her sudden interest and incredible skills in computing.
In Layer 05 Mika is subjected to many strange phenomena, culminating in being repeatedly told to "fulfill the prophecy" and hallucinating due to the influence of the Wired. She eventually loses her mind spends the next few episodes making random noises and hanging around pointlessly until Layer 09, when she begins to make modem noises. She disappears in Layer 10 along with the rest of Lain's surrogate family.
After the reset she is restored to her normal life.