The Lost Girl...? is a side quest in World 1 - Kanterbury Forest.
Quest Gameplay
In 1-6 Deep Ancient Ruin, north of Cecil's performance the player will see a sign warning about a vampire. If the player continues walking north, they will encounter a red-eyed girl. She will tell the player that she is lost and wishes that the player would escort her out of the forest. A charge attack icon will appear right below her as soon as the player turns their back on the girl. If the player turns around the face her, she will be surprised, and the attack will stop charging. If the player lets the attack gauge fill up, she will attack the player, and the player will pass out. When the player wakes up, they lose a portion of their health, and the girl is nowhere to be seen.
The player can escort her out of the forest, turning around occasionally so she doesn't attack the player. This is ill-advised, however, as once the player reaches the exit, she'll seem very ungrateful even though the player helped her. If the player head back to the spot where they first found her, she will show up again. If the player decides to interact with the girl again, they will question why she is here again, but she'll act like they've never met the player before.
Instead of escorting her to the exit, the player should escort her back towards the beginning of the stage. At the beginning, there is a big open area, where there is a light coming to the ground. Walk through the lit area and, due to the sunlight, she'll start screaming in pain before finally attacking the player. The player has to fight her as a mini-boss. After defeat, she'll run away from the player and escape into the forest. The player should then return to where they first met, where the girl launches an ambush against the player, who must swipe left to dodge her attack, otherwise they must tap rapidly repeatedly to free themselves of the girl's attempt to drink the player's blood. After failing to ambush the player, she will decide to join the player and their party, completing the quest.