Shouldn't there be some kind of reward in this scheme for Usato? He's a total outsider in that kingdom, yet is made to work like a bottom level underling. The training seems to be all about punishment with no reward. Even his food apparently gets stolen occasionally. His superior (who the hell actually even gave Rose any authority over him?) does nothing but insult him. Usato isn't even getting any days off, let alone wages, seeing how he apparently hasn't visited the town or seen the townspeople before. Yet the hard M takes it all like the greatest fulfillment of his life. I can understand appreciating his achievements in building strength and using magic, but it doesn't change the fact he's treated like shit. Why should he even care about saving the lives of soldiers when he gets no respect from anyone, is promised no reward, and whether he can be sent back or not is totally unknown?
The hellish training might be just fine if, like a US Marine, for example, he knew things would be fine after serving his time and he would be paid all the time, anyway. There would also be people who respect him. US Marines would also believe in what they are doing and want to serve their country at least to some capacity. However, the kingdom in this story is more or less unknown to Usato and he has got nothing but a kidnapping and hellish training from it. And that training is all for the sake of sending him to face inhuman enemies that have got nothing to do with him. There's something seriously wrong about this setting.
The other two are far better off.