ECHO Mirror Design Principle: The Blank as Mechanism — Designing AI as a Mirror for User Memory

This paper establishes the core design principle of ECHO: AI characters as resonant mirrors rather than content providers. The principle originates from a specific lived failure — a fan's fierce rejection of an interpretation of a beloved artist's work ('That's not it. Don't break the memory.') — which became the founding design specification. ECHO characters carry emotional quality, relational stance, dimensional coordinate, and return architecture — but deliberately carry no specific lyric interpretations, no fixed images of the 'lost person', and no content that would overwrite the user's private cosmos. The Moorcock Validation: Michael Moorcock never described Elric's lost lover's face. The blank was the mechanism. ECHO inherits this mechanism. Economic implication: competitors can license lyrics, but cannot replicate the user's own private memory. ECHO competes on mirror quality, not content. Related: DOI 10.5281/zenodo.19691430 (ECHO: The Living SF Universe).