We don't know the power forward of Rakuzan High yet. The player that appeared during the Rakuzan – Shūtoku match occupied the power forward position, but he was a so-called "faceless character", unnamed and of little importance. He however, is speculated to be only a substitute for the actual regular power forward of Rakuzan High. This hasn't been confirmed however and there also isn't any sign that there is a "mysterious fifth player" on the team.
The power forward was recently revealed to be chihiro muyuzumi and the "new 6th man"