If you want to think of a scenario, I've imagined the following:
1. In that case Alexander would somehow join the Order's forces, perhaps proving his skills during a siege in some city where he appeared and from there he helps the Order by developing strategies to face different types of monster girls.
2. Another possibility is that he was called by DL to help and face an entity that creates monsters of old, and to defeat this new threat they need someone like Alexander to win.
3. Alexander only appears in this world and decides that he's going to create his own empire there, but to become influential to both extremist sides, he's going to need resources, soldiers and territory and the story unfolds from how he achieves it all. , particularly I think this is the one with the greatest potential.