He was essentially a small Spirit Rune.
If a practitioner of Secret Martial Art asked Casio to modify and enhance his mediocre Secret Martial Art techniques, it would be a piece of cake for him. In just a moment, Casio could create a new Secret Martial Art based on the core elements of the original, yet more powerful and comprehensive, achieving an epic level enhancement. It might be second-tier, or even possibly first-tier, depending on how much effort Casio was willing to put in.
Therefore, for him, using a Spirit Rune to modify and enhance some Secret Martial Arts was meaningless and wouldn't bring much improvement to Casio.
As for something like the Southern Dipper Boxing Method, its core was too complete and its structure too stable, almost containing the entire boxing understanding of a top Saint Fist. Even with a Spirit Rune, it would likely be impossible to modify on this basis.
