Unity + Visual Novel Toolkit = 100% Free: https://assetstore.unity.com/packages/templates/visual-novel-toolkit-free-9416

Then there's also Ren'py https://www.renpy.org/ which is arguably  the most popular visual novel engine, therefore would likely be the easiest.  Easiest in the way that if you run into any issues, a simple google search will bring up a solution to an issue you're having that likely someone else would have also already had before. 

Hey don't go with unity or ren'py if you don't want to be forced to learn a little bit of code and ALOT of trail and error.

tyrano builder is the best for people that are artists and not coders. 

It has an easy to understand drag and drop interface, everything is color coded an in it's own little box.

It has issues like you can't make anything over a gig without manually packing it or when adding too many assets at once, because it will crash.

I chose tyranobuilder over the more popular and free engines because of it's ease of use.

Check out my demo if you want ....a demo.  LOL

Search, "The Surfeit.'

GOOD LUCK!