Artists retain copyright, but...

companies also retain copyright. Now, I'm not entirely sure how it works in Japan, but you can usually get info from one or the other as to how to secure rights for use. Even in the event of a band, the name under which they copyright their music is still registered to one or more individuals in the band, and they're supposed to keep the info current.

In the case of Matina, I think Kisaki retains rights as not only producer but company head, so starting with Under Code is a good idea.