The Anime Lad
A young orphan living on the streets of Shibuya stumbles into a fantastic world populated by beasts. There he meets Kumatetsu, a powerful warrior searching for an apprentice. Kumatetsu takes him in and begins training him. Their lessons include martial arts and a considerable amount of arguing over the instructional method.
Gigguku
The boy reaches a whole world of beasts and somehow finds the one mentor as stubborn as he is. Kumatetsu can fight, but teaching is another problem. Their training starts with bickering and bruised pride. That rough beginning gives the found-family premise real sparks before either of them can admit what the arrangement means!
Father's Basement
A lonely child finding a surrogate parent in a fantasy world is broad material. The useful choice is Kumatetsu. He is a capable warrior with rough edges and no obvious gift for teaching. His new apprentice is hardly compliant. Their constant conflict makes the bond something that must be built through training instead of granted by the premise.