Konosuba: God's Blessing on This Wonderful World!, Vol. 6 (light novel) Princess of the Six Flowers

After being invited to a royal banquet, Kazuma meets the young Princess Iris-the little-sister type he's always dreamed of. Enthralled by his tales of derring-do as an adventurer, she whisks him away to a fairy-tale life in the castle. Kazuma hopes it'll last forever, but then he declares he'll capture an infamous thief...