This is the secret origin of Mabel's mirror twin, Lebam. All characters belong to Disney, not me.
The Life of Lebam
"Mabel Pines, you have made me the happiest boy in the world," said Gideon.
He hugged her and pushed in close, using his right hand to clip a bit of her hair, and fibers from her llama-haired sweater. He let them fall into a small vial.
Mabel could feel something odd was going on, but she wasn't sure what. "Are you sniffing my hair?"
Gideon returned to the room full of clothing where he had given Mabel a makeover. He got out his Book 2 and reviewed the page for "Mirror Doppelgangers."
Standing to the side of the full-length mirror that had recently reflected Mabel's image, he got out the fibers. He pressed them to the mirror and chanted, "Speculum gemini, speculum gemini."
The fibers stuck to the mirror, where they grew and expanded into a complete mirror image of Mabel in her llama sweater. The image stumbled out of the mirror and into the room.
"Gideon? What's going on? Last thing I remember, we were sitting on top of your warehouse. Now we're back in your makeover room. I don't have time for any more makeovers today..."
"You're not who you think you are," said Gideon. "You're the mirror doppelganger of my darlin' Mabel. I think I'll call you Lebam, which is Mabel spelled backwards."
"You're such a kidder," said Lebam.
She moved in to give him a hard playful punch on the shoulder, but he touched the gem on his bolo tie with his right hand. The gem glowed green and her left hand was caught in a field of magic.
"Don't touch me without permission," said Gideon.
"What are you doing?" said Lebam.
"Magic. With this amulet I can make things move," said Gideon. He released her hand from his telekinetic grip.
"That's so awesome," said Lebam. "May I try it?"
"No. I don't want you turnin' its power against me," said Gideon. "Besides, it's breakable. If it hit something hard it would shatter, and I can never make another one."
"All right, just asking," said Lebam. "I need to go home now."
"You can't. I have something for you to do," said Gideon.
"Like what?" said Lebam.
"We're going downtown, and we'll take some photographs and do an interview for my self-published fan magazine," said Gideon.
"Why?" asked Lebam.
"If I build up people's expectations of romance between Mabel and myself, she'll find it much harder to say no to another date, and another, and another, and eventually my proposal of marriage," said Gideon.
"Marriage? You can't get me to marry you. I don't like you that much. I'm starting not to like you at all," said Lebam.
"The real Mabel is falling for me, I know it. You're her mirror opposite, so naturally you hate what she loves," said Gideon.
"Don't be silly. I'm the real me," said Lebam.
"Look at yourself in the mirror," said Gideon.
Lebam looked, and said, "I don't see anything different. What am I supposed to see?"
"Look at your sweater, and look in the mirror again," said Gideon.
Lebam looked. The mirror-reflected llama faced left, and so did the one on her sweater. "That's weird, these clothes don't reverse in the mirror."
"My book said that's how it would work. Since you're already a mirror reflection, the mirror doesn't reverse you. At least you have a reflection. That's good, or you might not show up in photographs," said Gideon.
Lebam looked behind the mirror, expecting a projector or some other trick. She saw nothing unusual.
"This is... this is real, isn't it? I'm not me. That means I've got nobody – no home, no friends, no Grunkle Stan, no Dipper..." said Lebam.
"Nobody but li'l ol' me," said Gideon in a sinister whisper.
"What have you done?" asked Lebam, getting angry. "You have to send me back to the mirror world where I belong."
"Do the magazine interview for me, and we'll see," said Gideon.
"Do it now," said Lebam. Like one of nature's greatest warriors, she lowered her head and charged him.
"Stop!" said Gideon.
Lebam froze in her tracks. It wasn't like the green glow that held her before. This time, her muscles simply wouldn't obey her, no matter how much she struggled to move.
"I forgot to mention that the amulet gives me a li'l bit of mind-control, too," said Gideon. "Since I summoned you, I can control your actions completely."
Lebam stopped fighting, hoping that she could get away later.
"All right, I'll do the interview, but I won't lie for you," said Lebam.
"Just smile and let me do all the talking. And here, wear this sweater," said Gideon.
Gideon pulled a plain light-blue sweater in her size off one of the hangers and tossed it to her.
"Plain? I don't do plain," said Lebam.
"You can't be seen in the llama-hair one since it's backwards from hers and people might remember. And besides, this one won't clash with my suit," said Gideon.
Lebam put the sweater on and went downtown with Gideon. She got her picture taken with him for the Gravity Falls Gossiper by Toby Determined, and she did the interview, smiling and nodding and hating it. Gideon ordered a mass printing of his new fan magazine, 'Gideon & Mabel', to be distributed all over town for free.
Afterward, Gideon took Lebam to his warehouse at 412 Gopher Road.
"Now you'll let me go... you promised," said Lebam.
"I don't know how to send you back. Anyway, it might be useful to have you around," said Gideon.
"What? Why?" asked Lebam.
"While Mabel is not yet mine, if I don't want to wait... you'll do as a backup," said Gideon.
"You'd cheat on her? You're despicable," said Lebam.
"It's not cheating, it's practice. You're a copy of her, my copy. But for now, I've decided I want to save myself for the real Mabel," said Gideon.
"Good," said Lebam. "That works for me."
"So I'll keep you here, locked up in my warehouse," said Gideon.
"No!" said Lebam.
"I can't have you running off to tell Mabel. It would spoil our big date tonight," said Gideon.
"She'll see through you soon enough," said Lebam.
"I'm going now. My Li'l Gideon dolls will keep an eye on you," said Gideon.
"You have dolls here? May I at least play with them?" asked Lebam.
"Don't touch the merchandise. The dolls are magic – they can talk, and they'll tell me if you mess with them," said Gideon.
He left to prepare for his date. Once he was gone, Lebam tried the door. As expected, she was locked in. She opened the top of one of the doll crates.
"Li'l Gideon dolls? If I play with you, you won't tell on me, right? I'm a fun person... a copy of one, anyway," said Lebam.
The dolls shook their small, big-haired heads. "No playing."
"We've got something in common. It sucks to be a copy of someone, don't you think? Not to mention being treated like a slave. We could get to be friends and take our minds off our troubles," said Lebam.
One of the dolls pulled the string of another, and a creepy laugh came out.
"Okay, I get it: you're as evil as he is," said Lebam. "Fine, I don't want to play with you anyway. I'll be in Sweatertown."
She put the lid back on the crate, then sat in the corner and pulled the plain sweater over her head.
She thought of Gideon sweet-talking her other self into another date. She whispered, "Please don't let him fool you, Mabel."
As she said the name, she felt a connection. Something felt different about the sweater she wore under the plain sweater. She felt the front of it and realized it was her purple one with the pink cat face. The sense of being in touch with Mabel was comforting. She allowed herself to doze off.
Gideon came back from the date radiantly happy. As he had hoped, the newspaper and fan magazines had built up the interest of the whole town. When he asked Mabel for another date, everyone had gathered around her and pressured her into saying yes.
While he was gone on the second date, Lebam tried saying "Mabel" again. This time she got a pink sweater with a red and purple butterfly. It came with a pink hairpin, which Lebam took out of her hair and put in her pocket so that Gideon wouldn't find out about her clothes-reflecting power.
It was terribly boring in the warehouse. Gideon was so caught up with Mabel that he seldom remembered to bring Lebam food. She survived on Li'l Gideon brand bottled water and paper packing material. Apparently being partly created from llama-hair gave her a llama's digestion.
Gideon was radiant again after his second date. But he came back the day after the third date furious.
"Dipper Pines has come between Mabel and me," he snarled. "He'll regret that to his dying day – which just happens to be today."
"No! Don't hurt Dipper," Lebam said.
Gideon ignored her words. "Maybe I'll have you show yourself just before I finish him. The last thing he sees will be his sister watching him die and doing nothing to help."
"It's not going to happen," said Lebam.
"We'll see about that," said Gideon.
He placed a call to Toby Determined, promising him the phone number of Shandra Jimenez if he would call Dipper and lure him to the warehouse at 7:00 that night.
"No, I won't give you her number over the phone," Gideon said. "I don't trust you. I want to hear you make the call in person."
After he hung up, Gideon told Lebam, "Dipper dies tonight. And then I'll be taking advantage of that backup plan we discussed. I find that violence excites me."
"You...!" Lebam began, clenching her fists, but Gideon cut her off.
"Be silent and stay back." Then he was out through the warehouse doors and she heard them lock behind him.
Lebam waited a few minutes for him to leave, and then she pulled out the pink hair pin she had kept and began working on the lock. She had to get away, had to warn Dipper. It was tough to pick and the pin kept slipping. It seemed like hours before the lock finally gave way.
Lebam took off the rather dusty blank sweater and said "Mabel" to get a new light-blue sweater with an umbrella on it. She threw the old sweater onto the top of a crate. If she met Gideon on his way back, she had half a chance of making him think she was Mabel.
She began running toward the Mystery Shack. There was a chance she could intercept Dipper, if he didn't take a side trip into the woods to collect interesting items to show Toby Determined. But of course he would; Dipper was anything if not thorough.
She finally got to the Shack. No sign of Dipper; he must have already left by now. She heard Mabel talking with Wendy on the front porch about breaking up with boyfriends, but there was no time left to rush in and explain everything. She collected Mabel's bike and helmet from near the back porch, where Mabel usually left them.
"I know what I've gotta do. Thanks for talking to me, Wendy," said Mabel.
Lebam moved into the shadows at the side of the building as Mabel walked by.
Mabel was muttering to herself, "It's too late to go to Gideon's house tonight. I'll break up with him in person tomorrow."
As soon as Mabel was out of sight, Lebam pedaled out past Wendy, who wasn't paying enough attention to notice anything odd about "Mabel's" rapid return.
Lebam rode up the hill toward the warehouse, worrying about what she was about to try. She had to pull this impersonation off successfully, or Dipper would die and she would wind up back in Gideon's power. She hesitated, but she couldn't live with herself if she just rode away and left Dipper to his fate... she had to do it.
Arriving at the warehouse, Lebam peeked in the window. There was Dipper, the brother she remembered so well and had never met, suspended in the air by Gideon's telekinesis and being threatened with a large pair of lamb shears.
"Gideon, we have to talk," Lebam said, pushing open the warehouse doors.
"M-Mabel. My marshmallow, What are you doing here?" said Gideon. The shears dropped to the ground.
"I'm sorry Gideon, but I can't be your marshmallow. I needed to be honest and tell you that myself."
"I-I don't understand," said Gideon.
Dipper was choking in midair. "Uh, Mabel!? This probably isn't the best time to be brutally honest with him."
Lebam knew she had to throw Gideon a lifeline to get close enough to do what she needed to do.
"Hey, we can still be makeover buddies, right? Wouldn't you like that?"
Gideon's eyes grew wide. "Really?"
She was close enough now. Lebam grabbed the amulet with her dominant left hand and tore it from around Gideon's neck.
"No, not really! You were like, attacking my brother, what the heck?" said Lebam.
Lebam tossed the amulet to Dipper, who taunted Gideon that he had no power without it. Gideon tackled him, and both of them smashed out the window and fell toward the ground far below.
The amulet landed at Lebam's feet. She had only seen Gideon activate it with his right hand, so that was the hand she used to pick it up. She focused her will, and found she could control the amulet easily. She swooped out of the window and caught the two falling boys just before they hit the ground.
Lebam floated down to the ground, holding the amulet above her head.
"Listen Gideon, it's over. I will never, ever, date you."
She threw the amulet hard against a rock, and it shattered in a puff of green smoke.
Gideon said, backing slowly into the woods, "My powers! Oh this isn't over. This isn't the last you'll see of l'il ol' me!"
Dipper said, "Thanks for the save, Mabel."
"No problem," said Lebam. "You don't look in very good shape. My bike is up there, so I'll get it and ride you home. And can I borrow your cell phone? Mine's out of juice."
"Sure," said Dipper, handing over the phone.
As Lebam walked back up the hill to get the bike, she texted to Mabel, "Stay away from Gideon. I saved ur bro's life from him 2nite. U can get the credit."
Mabel texted back, "Is this a joke Dip?"
Lebam texted, "I'm Lebam ur mirror double. Come out when I put ur bike away and take my place."
Once Dipper was safely home and everyone was asleep, Mabel came out to talk to Lebam, who was hiding in the woods.
"This is so cool," Mabel said. "You're like the identical sister I never had but I sometimes wished Dipper had been. Thank you for saving him."
"You're welcome," said Lebam.
Mabel gave Lebam a big hug.
"What will you do now?" Mabel asked. "Go back to the mirror world?"
"I don't know how to go back," Lebam said. "I guess I'll have to go away somewhere and try to start a new life."
"Stay here with us," said Mabel. "You can share my life. We'll swap places and fool everyone. It'll be super duper fun."
"I love super duper fun. But only if you can keep me a secret," said Lebam. "If not, Gideon will try to get me again, or your Grunkle Stan will want to use me for a sideshow."
"You got it, twin sister," said Mabel.
Author's note: credit for the idea of Lebam goes to The Sqoou, who published theories about her on You Tube.