"I Was Thrown Into a Hero's Party as a Mage, But I'm the Only Commoner"

Romie, a mage of the Magic Tower, is ordered by her superior, the Tower Master, to accompany a hero party and defeat the Demon King. It's a request that Romie cannot refuse. The hero is a person from another world, the holy knight is a marquis's son, the saintess is Her Highness the princess—wait. I'm the only commoner? There's no way Her Highness knows the prices at restaurants, and the marquis's son doesn't understand how to rent lodgings. The otherworlder is cooperative, but their senses and common sense are completely different— Romie sighed. "The burden on me is just too much!" ※This is also posted on other sites. ─────────────── ※I mistakenly categorized this as 'Otherworld Transfer' thinking there was a summoned hero. If the protagonist isn't the transferred one, it should just be high fantasy. I've unchecked that category. My apologies!