Seishirou can recite 1,529 incantations by heart. Of those, 463 were taught by his mother. Memories of Setsuka are grayed and unhandled, but her voice rings clear enough to shake off all the mental dust. She was always singing. Be it a repeated verse of a spell or an entire invocation, Setsuka would set them to an improvised tune and sing to her camellias, sing to her morning tea, sing lullabies to her son. It is in this manner that he learned. Thus, whenever Seishirou recites one of those 463 incantations, he can almost hear that imaginary music and it takes some effort not to sing along to his mother's voice in his head.
Subaru has never been very good at reciting incantations. When he speaks the words of a spell, they tumble about in his mouth and trip over his lips on their way out. This is not acceptable. Every word of a spell is important and must be clear and definitive. Flubbing even one syllable could change the entire intent of an incantation. It has happened to Subaru before; one time, he was attempting to exorcise a haunted rocking chair and one mispronounced "ha!" later the whole thing was in flames.
As a result, Subaru is constantly whispering spells under his breath, carefully enunciating each word in practice. This drives Seishirou absolutely insane.
"Be quiet," he says impassively, but Subaru merely chucks his toast at him from across the table and continues on uninterrupted.
"Stop it," he murmurs, lying sprawled across Subaru's chest (in this position, Subaru is least likely to attempt sneaking out of bed). Subaru ignores him, the breath of each enunciated word blowing Seishirou's hair into his face.
"Subaru-kun," he pleads without pleading, the honorific almost but not quite a whine. Subaru finally pauses, but only for a moment. When he resumes his chanting, he is no longer whispering but yelling. The words are spoken with perfect clarity and the ofuda careen towards Seishirou with deadly aim.
Seishirou retreats to the top of the bridge's suspension cables in order to evaluate the battle. He begins to recite his own spell and unexpectedly finds that his mother's sweet singing voice has been replaced by Subaru's soft whispers. He is given no time to decide whether this is a pleasant or unwelcome change.