There is one thing we know about Mika, it is that she is not Lain's real sister. Mika is only loosely connected to the Iwakura family. She spends much of her time away from them. Mika thinks Lain is very strange and generally has very little to do with her.
Despite all of this, she does attract the attention of those who want to weaken Lain's powers in the Wired. Mika becomes the first target chosen by the Knights. They wrongly assume that the Iwakura sisters share a close connection. It is their plan that by taking control of Mika using their "KIDS" technology that they can discourage Lain from finding her true mission. (See Who is Lain Really? for more information on this.)
The Knights capture Mika's soul and trap it in the Wired. Her physical body remains in the real world as a rapidly declining shell of her humanity. Evidence of this action is as follows:
- The Knights Symbol appears around Mika when her soul
is captured in Shibuya square.
- In the KIDS episode, Professor Hodgeson explains to
Digital Lain that the use of the KIDS technology trapped the
experimental children's souls in the Wired.
- Mika arrives home and sees another version of
herself. The "just arrived Mika" at the door dissolves into
an energy mass - that is Mika's soul. Her physical shell body remains.
- The prophecy is "Hell is full of the dead, and the dead shall wander." Mika has executed the prophecy, i.e. her soulless body walks aimlessly around the house.