It is entirely possible to use a foil in battle. They were mostly duelist weapons and other items would probably be used in real combat, buuuuuuuuuut... I'd like to remind that Yui here is a hero who will be using a ring to get whatever weapon she desires, regardless of history or anything. If she happens to think a foil is the best weapon for her, then damn right the ring will make one for her! XD So from that point of view, historical accuracy and such things really matter nil. After all, the hero gets what she wants. If she has played this game which gave her the impression foils are awesome combat weapons or chooses one because she has fenced with one before... Then so it will be~!

I just felt I should mention.