Living Phantom
An extremely peculiar part of the Nine Shadows, the Living Phantom ability is a way to walk in someone else's shoes. Using a feeling of the target's personality, the ability wielder acts as if they are the real thing. When properly executed, the ability itself is in control, making the wielder feel like a spectator.
To use the ability, the wielder must be familiar with the target, having personally interacted with them. It uses subconscious impressions of the target, which is why personal interaction is a must. However, repeated interactions will end up reducing the ability's reliability, as the wielder's personal feelings will begin impacting the impersonation. These limitations make it a hard ability to put to good use.
Another restriction is that due to being based on subconsciousness, trying to steer the act will make its result less reliable. Akin to how trying to force a Tarot Cards laying will taint its results, trying to force the focus of the act will only ruin its accuracy. One's own biases will surface instead.
When properly used and combined with illusionary abilities, a wielder may manage to completely mimic the target. This can be useful for infiltration purposes, as long as the wielder doesn't try to force a situation. However, prolonged usage is ill-advised, due to the theoretical risk of the false personality becoming dominant. Furthermore, Wardstone barriers would dispel the illusion and expose the wielder.
Oh, that one looks fun! :D Should there be a person who has either multiple personalities or wears a mask when interacting with specific people, the Living Phantom will be a projection of the subconscious image of the copied person? And maybe a more general one. Is there a way to counter the ability or see through it?
You'd be copying the feel the person gives off, their vibes. That means you might partially pierce through a faked personality. If they actively had multiple personalities... Probably the surface personality and a bit of the original's vibes, I'd say. If you're familiar with the real person, it would be quite easy to go 'hold on, something's wrong here' when dealing with a disguised fake. If you don't know them at all, the only real counter is barriers and Sealstone. Paranoid places in the know might use Sealstone doorknobs. And there's also people that are skilled enough at Sensing that they'd realise the person is off, and on closer inspection realise the wielder is a layered person, so to speak.
Too low they build who build beneath the stars - Edward Young