This was fantastic. And even as a grown man, it had me emotional at quite a few points. Little Raiga making the choice to abandon any shot at a "normal" life to train and become a Makai Knight was a moment that needed to be shown, and it was great that we got to see another familiar face from the past return, that wasn't Zaruba or Gonza. Makes this feel like it is connected to the first two seasons after all. I really do hope that we still see more of the old cast pop up from time-to-time in flashbacks like this, I'd hate to think those characters are gone completely.
Like I said, I got quite emotional watching this, but most of it was the end credits with Mayuri and the picture books, AND the painting.
As far as flashback episodes go, this was almost up there with Makai Senki's episode of Kouga training at Kantai.