Why does Yasako have an encode reactor?
In a forum post asking the same thing you did, 


  So, where did Yasako's encode reactor come from? It can't possibly be from Isako's prior encode; Satchii always chased Yasako even when she was "clean." It must have been built into her glasses from the beginning by the Cyberpappy. Exactly why he felt the need to do this, I don't know.


this was the reply:


  It came from the kilabug (kirabug?) that Isako transferred while
  trying to save Haraken.