My first rant is on how the promise girl could not be Marika.

We know that she first met Raku in the hospital but it is clearly stated that Raku first met the promise girl on a Hill at night.

To add to this with Marika's condition as a child she didn't really go outside. We see at the very begging of the anime Raku with the promise girl outside. I even think at one point in the manga Marika stated she couldn't go outside the hospital.

Overall Marika being the promise girl is very unlikely.