Hansel and Gretel are the titular characters of the classic fairy tale of the same name by the Brothers Grimm. They are two little kids who are brother and sister. Lost in the woods, they become friends with dwarfs before they are manipulated and kidnapped by an evil witch. They are saved with some other children by the dwarfs.
Hansel and Gretel appear in Once Upon a Time, portrayed by Quinn Lord and Karley Scott Collins, as inhabitants of Fairytale Land. When their father went missing, the siblings sought out to look for him and ran into the Evil Queen who offered to help if they help her. The Queen asked them to enter the home of the Blind Witch and steal a bag while the witch is asleep. When Gretel asks why she doesn't go herself, the Queen claims that the house has a spell that keeps out adults but not children. She also warns them not to eat anything. The children manage to sneak in the witch's house but Hansel eats a cupcake, awakening the witch. But they are able to get out of the gingerbread house alive and deliver the bag to the Evil Queen, which had contained the Poisoned Apple. She offered to let them stay with her, but Hansel and Gretel declined and tell her to find their father instead. The Queen became angry and sent them back to a forest, and it is revealed that the Queen was the one who kidnapped their father. When the Woodcutter informs her that his children refused to stay with her because "family always stays together", the Queen tells him that they can be together if they can find each other, forcing the Woodcutter to search for the children as well.
In Storybrooke, they are Nicholas and Ava Zimmer, the 12-year old twins of Michael Tillman, the auto shop owner. Their mother Dory passed away and have been living on their own ever since. They are first seen shoplifting for food and frame Henry Mills for it. Emma Swan investigates what happened, and goes on to interrogate the twins. When she learns they are orphans, Emma sympathizes with them since she also has no parents. She goes on to look for them using a compass Dory used to keep, and goes to Mr. Gold to ask him if someone had purchased the item before.
Mr. Gold gives her the name of the man, Michael Tillman. He denies the children are his, but recognizes the compass. He wants nothing to do with them especially since he was unaware of their existence until now, and cannot accept the responsibility. If Ava and Nicholas' father continued to reject them, they would be taken to Boston to be placed into separate children's homes. Emma calls Michael to fix her car before they leave. Ava notices that her compass needle is moving. Tillman arrives in his tow truck and realizes that the children are his. He hesitates but then agrees to be their father, and then goes to his children.