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...
Well! That is very interesting and would make it more fitting, I guess I was indeed not seeing all the versions ( I refer to my previous answer to kuromiko)
Thanks for the spoiler ╮(￣▽￣)╭
Where to read the Web novel?
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.