IIRC, Balthamel was an anthropologist in the AOL. I'm not sure why he was supposed to be one of the top leaders of the Shadow. Perhaps he was just strong in the OP and happened to be at Shayol Ghul when Lews Therin sealed the Bore.
We know that Balthamel loved pleasures of the flesh, and after his transformation into Aran'gar, she became even more decadent. She was more interested in enjoying herself than furthering the Dark One's plans. Even her alliance with Graendal was more a way to get into Graendal's bed.
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