What makes a Seraph different from a Doppel?
From what I understand, a Doppel is not exactly a “permanent” summon. Without going into spoilers on why Doppels exist or how a Magi gets one- Doppels seem to be more akin to a super-powerful “attack”, kind of like early final fantasy summons.
Seraphs are more akin to say, Stands from Jojo’s bizarre adventure, though to be even MORE accurate, one could say they are super-powerful “unique” familiars that can be obtained by a magical girl in Rebirth that are directly linked to their master and can grow and change over time and through various experiences (though mostly combat).
Seraphs are also conscious, living entities with a bond to their “master” and are free to operate autonomously apart from their master when not needed or not in combat, leading to a Seraph developing it’s own personality, habits and traits as it grows and evolves alongside it’s master- Making them kind of almost like the more “classic” idea of magical girl pets or familiars as well... Though likely far more deadly.
Seraphs don’ t really talk, however, they just kind of jitter, coo, moan, etc. in their own strange, unintelligible language.