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She was sent to prison for trying to close down an amusement park and eventually released early and then worked for Mr. E. During this time she befriended Velma, gaining membership into Mystery Incorporated as a mole, temporarily replacing Daphne Blake. When Daphne returned she was forced to leave, and continued working for Mr. E. She was sent to Mount Diabla to retrieve a piece of Planispheric Disk, but handed it to the gang because she couldn't betray Velma; her only friend. Since then she went into hiding, still managing to help Mystery Inc. in an elaborate scheme to get Mr. E's pieces of the Planispheric Disk which she stole personally.
Her brown hair is usually very messy, in loose, curly waves. She wears yellow, low hanging sunglasses with red rims.
Originally she was very competitive with Velma and they had a strong mutual dislike. This changed, however, when they bonded after both working for Mr. E.
Skills and abilities
She is good with computers, and a brilliant amateur chemist.
She had competed with Velma in the science fair circuit more than once, and lost.
Scooby-Doo! Mystery Incorporated
She went on an exchange student trip to China, where she had an allergic reaction to the gluten in the noodles.
She attacked people at Creepy Spooky Terror Land in the guise of the Manticore in order to shut it down so she could use the steel in the rides to make super-helium and get rich. (Her father, Winslow Fleach, owned the amusement park).
She claimed she was released early from prison because she improved the computer system there to be more harsh. This may not be the whole truth; Mr. E most likely had something to do with it as well as he was the one who bailed her out. (The House of the Nightmare Witch)
She was kicked out of the gang after Daphne came back. Upset, she went back to working with Mr. E. (Web of the Dreamweaver!)
Mr. E sent her to Mount Diabla to search the Spanish Galleon there for its piece of the Planospheric disk. She disguised herself as the Dark Lilith to lure the gang to the mountain and get past the traps for her, but gave them the disk piece after Velma caught her. (Night on Haunted Mountain)
Velma Dinkley later enlisted her to help steal the Planosperic Disk pieces that Mr. E had acquired. Marcie's job was to infiltrate Mr. E's vault and grab the pieces while Jason Wyatt kept him distracted with the Krampus. (Wrath of the Krampus)
Captured by Pericles, she was used as a hostage to force the gang to open the inter-dimensional portals for him. She managed to grab Pericles as a hostage after the Air door was opened, and heroically sacrificed her life, being shot many times by the Kriegstaffebots, to give them a head start.
When the gang created an alternate timeline with no influence of the Evil Entity, Marcie was alive and she and Velma are a team and win every year at the Tri-state Olympiad of Science. They are best friends and Marcie refers to Velma happily as "That's my girl."
- Scooby-Doo! Mystery Incorporated
- 116. Where Walks Aphrodite (cameo)
- 118. The Dragon's Secret (flashback)
- 121. Menace Of The Manticore
- 202. The House of the Nightmare Witch
- 203. The Night the Clown Cried II - Tears of Doom!
- 204. Web of the Dreamweaver!
- 205. The Hodag of Horror (cameo)
- 208. Night on Haunted Mountain
- 213. Wrath of the Krampus (cameo)
- 224. Gates of Gloom (unseen, but present inside a box)
- 225. Through the Curtain
- 226. Come Undone
- Linda Cardellini played Velma Dinkley in the live-action movies, Scooby-Doo and Scooby-Doo 2: Monsters Unleashed.
- She is allergic to gluten. (The Dragon's Secret)
- The super-helium may be a reference to the old Disney movie with "Flubber" gas, Son of Flubber.
- She is the second character in Scooby-Doo! Mystery Incorporated to wear two disguises, the first being Alice May.
- She is one of the few characters to have been killed in a very gruesome manner. Being killed by a gatling gun.