
So in the end, the voice of the Great Sage should have been male after all, this last chapter just made me think that again... I still cant understand why they gave him a female one in the anime. Tho I also dont know why they called his evolution Raphael, if he is the “Wise King”, Solomon would have been more fiting.

Raphael is the name of one of the 7 Angels that stand by the throne of God. He is know for bring knowledge and new ideas. He also known for healing mind, bodies, and spirits. While Solomon was just a human.
Also both Rinmaru and the Great Sage are genderless. So having Rinmaru have a male voice and the Great Sage a female voice is most likely to represent that.

This is a light spoiler from the web novel: There are 2 competing sets of ultimate skills, the 7 heavenly virtues and the 7 deadly sins. Each virtue has a designated angel while each sin has a designated demon. This is all theological, which I assume the author used as a basis. (except for temperance and humility, which were replaced with hope and justice).
Raphael in ancient texts have 2 very prominent roles, as a conveyer of knowledge and as a healer. Which makes Raphael a great name for a skill that provides Rimuru with knowledge and brings back to life his dead subordinates.

Hmmm... maybe I was seeing it from a very catholic version, where Raphael is one of the 3 archangels and his field was more of healing and traveling, not so much wisdom.
Tho I do stand by my preference of character in the sense that you are right Solomon is just a human, but I think it would be cool the parallels between Rimuru being "just a slime" and yet how much power he has plus the whole monster loyality and literal demon sumoning as parallels with Solomons' demons. Tho I guess that would be with Rimuru himself more than with the Great Sage.
I would agree with both of them being genderless, but I would also maintain my incomformity with the voice choice, cos with Rimuru you could hear a little bit of a female voice (cos his slime VA is a girl) but it was also very boyish, but the Great Sage was an outright female voice when if I had to pick one of another I would still choose male cos the names it is reciving...

So you can see the art on those two blank pages with only the texts: https://tmofans.com/viewer/5c27b9b61a59f/cascade
This fucking idiot...