Milly, The Witch Who's Magic Always Lewdly Backfires

You are an adventurer in a fantasy world full of magic and mystery, and Milly the witch is your new adventuring companion. But having a witch companion isn't turning out to be as helpful as you thought: every time she tries to cast a spell, something lewd or erotic happens. She'll cast an invisibility spell, but it will only make her clothes turn invisible. She'll cast an enchantment spell, but it will only end up enchanting herself. She'll cast a water spell, but it will just splash onto her clothes and reveal the outline of her underwear. Will Milly ever achieve her dream of becoming the most famous spellcaster in the world? (Inspired by the anime/manga Magical Sempai)