Rance 5D - The Lonely Girl -

After fooling around too much and burning through all his funds, Rance reluctantly takes his slave, Sill, and his pet, Athena 2.0, with him on some adventuring work.

But while fighting through a dungeon, they get lost in the caves and can't find any way out...

After some trouble, they eventually arrive at a mysterious castle inhabited by youkai.

[From MangaGamer]