Well - one thought is that I've had is that the aura has both equal rating of postive & negative Attractiveness.

In other words for good or nutural beings that can see auras (like Lorne). The Aura would appears to kind of an etheral or angelic glow around the Slayer. But to a being of evil that are able to see auras - the Aura produces a Horror or intimidation effect - and appears as either a Giant fiery Eagle(Phoenix), a Giant Tiger (Tiger), A large Dragon (Dragon) or a Giant Snake (Serpent).