I've always felt that the author dropped the ball by writing Sein out too early. For the chapters and episodes where he's in the party, he's like the salt that brings out the flavor of the other characters. Any time Sein is having a one on one chat with Frieren, Fern, or Stark, they open up and reveal a lot more about their qualities and character. Sein gets plenty of development of his own in this short time.

But having him for only 4 episodes just seems wasteful. If his departure was written, idk, 10 episodes out, then it would have felt like we actually spent good quality time with him. And his journey would still be but a fleeting intersection with the others'.