Heh. This was fun.
Our MC loves dem Isekai tropes and is a goofball dork. The "heroine" is a sweetheart too. Probably should've spilled the beans, all things considered, but I'll let it slide since that's the premise's conceit.
I think the perfect excuse here would be to pretend to help her visualize things using references you share from your old world. Considering she'll literally only materialize things if you're around, I think it'll sell the ruse quite well.
Also, like, make a sword and shield for the two of you so you're properly protected.