Most people have a negative perception of one-dimensional personalities, and this is not without cause. As an illustration, consider Goku from Dragon Ball. When Goku appears on screen, no one will ever wonder what he will do next since his position in the tale is always clear. It's simpler to describe Goku as "flat" than "one-dimensional." They depict a distinct picture even if they are saying the same thing.
Flat characters don't have any form of character arc; instead, their "arc" is a flat line that runs from beginning to finish of the narrative. No one ever gets frustrated by Goku's lack of character development in Dragon Ball because the plot is relatively simple, but the most recent episode of
It's Incredible How Nice Rein Is.
The last episode left off with the party headed to the Mithril Mine to investigate why the flow of supplies had ceased. Then, in Episode 9, Rein and co. discovered that there were bandits in the mine, and one of them had to be a fellow beast tamer who had been using a hawk as an alarm system, circling above the entrance. Upon entering the mine, Rein's party neutralized the bandits and the beast tamer, who had managed to tame a behemoth, a monster, rather than a beast.
Rein killed the behemoth and captured the beast tamer, taking both him and his party back to town to be handled by the law. Days later, Rein discovered that the bandits had been shipped off to work as slave laborers which, for some reason, saddened Rein. Despite committing numerous illegal activities and trying to kill him and his friends, Rein still didn't want to see anything bad happen to the bandits. It isn't as though Rein should have killed the bandits (although he would have been justified in doing so), but his reaction to finding out what happened to them is starting to stretch the levels of believability in his character, and this issue is now becoming tiresome.
Rein Is Perfect for His Own Story, but Lacks as a Result
Beast Tamer is no narrative masterpiece, and that's okay. It's a story that knows exactly what it is and wants to be, and it works toward being those things in a reasonable manner. The issue is that Rein and his party feel flat and somewhat boring as a consequence. There's never any question about what they're going to do next, no matter what situation they're placed in.
Rein is a pure soul, always on the side of good, but it would be nice to see him deviate from this path occasionally. Small glimpses of this were shown when he stood up for himself against Arios, the leader of his former party, but none of it was shown when dealing with the bandits in this latest episode. If there were ever a time to get angry, now certainly would have been it, but it doesn't appear that Rein will change any time in the future.