I’ve been going back and forth for a while about whether my Sephiroth deck’s Party Leader should be Sephiroth, Planet’s Heir or Sephiroth, Fabled SOLDIER, and ultimately chose the 
option. Adding in the provides a lot more control as well as more surveil abilities. I also feel like getting out Emblem: Sephiroth, One-Winged Angel is less valuable as a Party Leader ability, since once it’s out you won’t need to try and transform Sephiroth again.
The goal of this deck is to try and cast Sephiroth, Planet’s Heir for as much value as he can get. With any luck you’ll get a good chunk of counters by the time he’s ready to be cast. Sephiroth’s Intervention is mostly for flavor, but by the time your Party Leader is on the board, targeted removal becomes very useful to further power him up.