there are various considerations to employ in understanding the 'hidden self'...I don't think it is determined by one planet alone...I think that the Sun and the first house relate to one's persona, or sense of Identity, while the Moon is more subconscious in it's expression...Neptune is about 'secrets' so this is also a contributing factor, and being rather ethereal in nature, speaks to our connection with our inner divinity. When we have the outer planets [Pluto, Uranus, Neptune] configured with personal planets in the chart, I believe this indicates a greater potential for discovering more about the inner facets of our being.
Look to the aspects of the Moon for indications of internalized challenges. Aside from that I believe that the 4th house represents the seat of the Soul, per say, as the collector of conditioning and emotional responses. These in turn will have a significant effect on our life into the future, but the contents often remain below the level of awareness until we have been awakened to the fact that we are more than just an external persona and identity. The 12th house also relates to things hidden. Planets below the horizon are also considered more subconscious than conscious.
Our outer persona is the interplay between the Sun and the Moon, which must then be channeled through the Ascendant. In terms of understanding the inner self, the work requires much self-analysis and honesty about our inner fears and weaknesses, for these are the things that have the most to teach us. Saturn
in the 4th represents such hidden fears: Uranus
therein suggests the need for freedom from past experiences that serve as internalized blockages
to becoming balanced and feeling whole. It is this 4th house dynamic then which should be the focus of study in determining more about the hidden self.