Board Thread:News and Announcements/@comment-4698889-20180724122634/@comment-6986530-20180906203031

Arth was based on an adult version of Len.

His song is sung by a Len (I know that you have a point with the Meiko singing Leonhart's song, but this is to be taken in addition to the point above)

We have had cases of a Vocaloid portraying both one character and their father (Kaidor and Kaspar) so the argument that he "shouldn't" be identical because he's Allen's father is invalid. Plotwise, it also makes sense as Arth was descended from an Irregular.

For the time being, he's a Len character.