Lucy Gringe is the daughter of Mr. Gringe, the Gate Keeper, and Mrs. Gringe. She has a younger brother, Rupert Gringe, who works with Nicko Heap in Jannit Maarten's boatyard, and who, like her parents, greatly disapproves of her relationship with Simon for most of the series.
She is in love with Simon Heap, who she met when they were both 14 at a ballroom dancing class, and who she almost married when they were seventeen.
She has brown hair and big brown eyes. Her hair goes all the way down to her waist, and she wears it in two plaits, with lots of ribbons braided in. She wears long dresses, often decorated with flowers she embroidered herself, and boots several sizes too big for her.
She can be very girly at times, and loves ribbons and pretty things. She can also be very tough and stubborn. Overall she is a very strong and determined character.
In Magyk, Lucy Gringe meets Simon Heap when he sneaks away from the rest of the Heaps. They try to get married, but the Suprieme Custodian captures Simon and interupts the wedding.
In Flyte, Lucy tries to run after Simon after hearing that he had kiddnappd Jenna. Gringe stops her by winding up the drawbridge.
In Physik, Septimus meets her on the outside path, when she runs away from home to find Simon. Simon sent her a letter telling her to wait for him, but she's tired of waiting. She doesn't know where he is, but decided to go find him herself. She takes one of her brother's paddleboats and sets of toward The Port. Later on, Wolf boy finds her by the river. She is very distressed because her boat is missing. Nicko had been looking for it, and had found it while she was sleeping. She goes to him and demands to have it back, but he ends up giving her his boat instead. Lucy is very relieved, but tries not to show it. She knows that her boat would not have been sturdy enough to travel all the way to The Port.
She arrives in the Port late at night. The next morning she buys several pies from Maureen, the owner of the Harbor and Dock Pie Shop. Maureen sees how cold she is, and invites her to sit by the fire. Lucy ends up working in the pie shop with Maureen in exchange for a bed in the corner of the shop. She asks everyone who comes to the shop if they've seen Simon, but none have. Finally she meets Stanley, who, after a long while, finally agrees to take her to Simon. They leave in early spring and head toward the badlands. By late spring Lucy and Simon are reunited.
In Quest Lucy goes with Simon to get a diamond ring. She gets one, but she and Simon come back to the landwurm burrow to find a crowd of Things that Merrin had summonded. They also find that Sleuth is gone, and Simon decides he must find Merrin and get Sleuth back. They also find a note from him telling them that they'll never get Sleuth back. Simon is outraged, and they set out for The Castle at once.
Lucy first appears in Syren when Wolf boy encounters her in the House of the Port Witch Coven. She had been kept prisoner by the witches for several days after they had captured her at the market. The witches offered her to Wolf Boy as Grim food, and he eventually accepted. It was clear that if he did not, they would just push her in anyway. The witches went upstairs for the Grim feeding, leaving Wolf Boy a small amount of time to come up with a plan. He told Lucy to scream, and while she did he fed two vampire cats to the Grim. Wolf boy cut off the tip of one of the Grim's tentacles, and then he and Lucy escaped through a hole Lucy had made in the cellar wall. When Lucy and Wolf Boy snuck away, they were chased by the witches (who smelled the Grim blood from the tentacle Wolf Boy cut off). Simon saw them, thinking Lucy ran off with another younger man. In truth, Lucy and Wolf Boy headed into Skipper Fry's boat that just set of for CattRock Light. There they saved the life of Miarr Catt and helped him to get back his Sphere of Light. They meet Septimus and others on the Isles of Syren and together they manage to trap Fry and his two servants into Cerys. She was later reunited with Simon again.
In Darke Lucy takes the port barge to the castle in order to deliver wedding invitations to her and Simon's parents. While at her parents' house she encounters Thunder, and she knows that Simon is in trouble. She sends him a message, but gets no response.
In Fyre, Lucy marries Simon. She also is the architect of the new Alchemie Chimney
In PathFinder, the first book of the TodHunter Moon series, Lucy works as a servant to the Lady and helps prisoners escape. Her's and Simon's son, William, is being held by the Lady and with the help of Tod, Oskar, and Fergie, she successfully infiltrates the place where all the prisoners of Oraton-Marr are being held and is reunited with William by the end of the book.