According to
http://beingnext.info/blog/bvcl-830/ +d is Daikoh Nagato (長戸大幸). However, I'm not sure whether this is an official Being blog or not (probably not as it is contains some chinese text and is registered in TW).
+D is also listed as an alias on his English Wikipedia page:
https://en.wikipedia.org/wiki/Daiko_Nagato
But the citations provided for the statement don't actually provide any information to back it up...