Either is not possible. First, there is currently no way to access information of the 3D world (e.g. where monster X is on your screen). Second, there is currently no good way for addons to transmit and receive data (e.g. your background story).
The only thing that is possible in this context is to make use of guild notes. So when a player from your guild appears you could show the guild note. Alternatively you could create an addon which stores Info for other characters you encounter, like a personal note on a per character basis.
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