afroakuma's Planar And Other Oddities Questions Thread VIII

[Tzardok]

So, this is my question. Not about this specific aspect, but the general idea. For example, Sharess is goddess of lust is considered to be an aspect of Shar, goddess of darkness. How is it possible, that both exist as characters or entities, have a physical form on Outer Planes and each with separate consciousness, but still one is a aspect aka a part of another?

Simple. Shar is a lying liar who lies. (I'm noticing a trend in those questions...) Shar does this thing where her church infiltrates other religions and slowly makes them believe that their god is just an aspect of Shar. This leads, if not counteracted, to Shar subsuming the other deity. Shar did this successfully with Ibrandul, a deity of caves and dungeons, and currently tries with the Chultan deity Eshowdow. Sharess specifically was saved from being subsumed and the infection of Shar worshippers in her cult destroyed through the help of Sune.
Shar isn't the only deity who uses this tactic. Set for example successfully subsumed the Faerûnian yuan-ti deity Sseth.

I mean, in Planescape cosmology deities are actually people, very powerful, but still people who behave like people, though in some cases their alignments are much more extreme then those of mortals. How can a person have aspects who have their own consciousness?

And this is your problem. Planescape deities aren't people. They are forces of (super-) nature that occassionally look like people. They can be in many places at once. They can form avatars and aspects. A deity can sense things miles away, hear every instance of their name spoken by a mortal, notice every event involving their portfolio in their area of worship, sometimes weeks before it happens. A deity's personality is formed by their portfolio and the beliefs of their worshippers, not the other way round.
A deity is not a person. It's a power.