Keeping Rare souls for future event

[Yup]

No it is the opposite, the character gets decided once you drop the soul in Eddie’s mouth

I understand that in theory.. but then.. why..

can I drop a soul.. see it's character.. get a game crash... relaunch the game.. the same soul is there again.. I drop it a second time theoretically giving me the chance to get any character again.... yet I get the same character?

This has happened to me more times than I can count. And not just with individual souls.. drop 10 Maker souls.. see the 10 characters... crash.. relaunch, same 10 maker souls result in same 10 characters. If dropping determines what character. there's no way 10 souls give the same result.

Sorry. From my own personal experience, I feel the character is decided when you get the soul, not when you convert it.