Never knew a schizophrenic hallucination could have a birthday.
I liken Midori’s character to a temptress, a character that tries to dissuade the protagonist from his goal with something enticing, like sexual pleasure (which she actually does use in the route). However, her goal in tempting Riki was not to goad him into accepting her as ‘Mio’, but to see if he could be strong enough to persuade Mio to give up her desire to disappear from the world due to her grief in forgetting Midori. Midori never intended to be a replacement for Mio, and she knew that there would be a point where Mio would forget her, because she was a figment of Mio’s schizophrenic mind.
If anything, I would compare her to a softer Kanata, the main difference being that while Midori had no intention of hurting Riki or Mio, Kanata goes out of her way to make sure that Riki and Haruka never succeed in finding her true lineage, even to the point of breaking them emotionally. Midori was also more focused on having Riki choose the lesser of two evils, because having Midori around instead would only hurt Riki emotionally, while trying to save Mio instead would require greater strength, and Midori was only preparing Riki for that greater challenge.
So, to Mio’s hallucination/twin sister, I wish her a very happy birthday! (pachi pachi pachi)