The only reason I could see you keeping the Scythe is if this Ralsei replacement doesn't want to hurt people, and therefore uses a weapon bad at combat.
I guess even that's questionable though.
If you choose a knife, I think you should go for a Swiss army knife, as they have a lot more utilities than just attacking. That would give a reason for Ralsei (or a Ralsei replacement) to use the knife, as he never likes fighting.