Wow, this story got a lot of good reviews and good ideas. Glad people like it. Anyway, this story has been on my mind for a long time ever since I watched how Harley Quinn became the Joker's lackey. Enjoy.

Disclaimer: I do not own Batman or Danny Phantom. They belong to the world, but they really belong to their creators.

Harleen Quinzel, a woman in her young twenties, was walking to the 'special' rooms of Arkham. She wasn't so bad to look at; a heart-shaped face, blond hair tied to a bun, blue eyes behind glasses and she was dressed in a red shirt and skirt under a white coat.

She was a recent college graduate who wanted to study the minds of the criminally insane, who she viewed as fascinating. So when she got this job, she was happy and excited. To study minds of the insane and find out how they view life.

She was here for almost three months and was assigned to one of the most mysterious patients in Arkham. Though it wasn't the infamous Joker, much to her disappointment, but she was happy to the patient she was assigned to.

"Hey there, Harley!" a voice exclaimed that caught her attention. She looked and smiled. "How are you?"

Her patient, a man named Daniel Fenton, aka, Danny Phantom, the lost Ghost Boy. He was a little pale with long, shaggy, black hair, blue eyes and a scraggy beard. Like all patients in Arkham, Danny was dressed in an orange jumpsuit, the code on his chest was 'A-13' and has an inhibitor collar on his neck.

He was sitting cross-legged on the floor in his cell. In his room was just a simple bed, a table, a bookshelf full of books and a sketching pad.

"I'm fine, Danny," Harleen said with a smile as she grabbed the wooden chair and sat on it.

The first time she met him, she thought he was just an ordinary crazy man. But the more she talked to him, she discovered how mysterious he was. He was born in a family of geniuses and inventors in Amity Park, the most haunting place in the world.

She learned that he was one of the longest patients in Arkham since he was in the building for six years, after the 'Nasty Burger' incident. She was saddened when she heard that he lost his family in the incident.

What she found fascinating was that he placed himself in Arkham, confessing that it was his fault the explosion happened. Many thought that he was delusional, so they brought him in. Despite being the most best behaved in the asylum, he refused to be released.

She found that to be interesting. Even though he seemed normal, the guards could hear him talking to himself at night, saying that he was sorry and promised to never become him, whoever 'him' is.

When she first talked to him, she asked him some questions. Some were normal while others were personal, but he didn't seemed to mind. From what Harleen could tell, Danny felt guilty for the death of his friends and family. Even after all these years, he was still in grief.

Harleen helped him as the two continued talking. She managed to break the armor around him and turned him into a cheerful person than he was before. She also enjoyed their talks. Soon, her visits were now friendly than business.

"So how are you?" Danny asked as he sat on his chair and leaned on it.

"It's been good," Harleen said as she removed her glasses and smiled at Danny. "Have you been good?"

"Good as a cucumber," Danny said with a grin. "It's boring here without you. I miss you."

Harleen blushed before coughing to calm down. The two then just spent the hour talking and telling stories. Harleen laughed as Danny told her a joke.

For the past week, Harleen has developed feelings for her patient. She has never been close to a man before, since she was more focused on her studies. She felt lonely and she knew Danny felt lonely as well.

To her, he was her kindred spirit, someone who understood her and she understood him.

When their session was over, Danny gestured her to be quiet. He then clasped his hands before white mist appeared. Her eyes widened when Danny opened his hands, revealing a ghost-shaped crystal the size of his palm that had a happy face on it.

"You have so much free time in this place that you could almost do anything," Danny explained as he saw her shocked face. He then gave the small ghost to Harleen, who smiled at the gift. "See you tomorrow."

Harleen looked at the ghost crystal before she hugged it as she walked out the room.

One Week Later…

Harleen sighed as she walked towards a different room. Just yesterday, the Warden said that since they were impress with Danny's progress, they reassigned her to their most difficult patient, the Joker.

In the past, she would have been excited, but then this means that she won't meet with Danny again. With a sigh, she entered the room and there she saw the infamous man in Gotham. Green hair slicked back, pasty white skin, yellow deranged eyes and a smile so big it would hurt. He was also dressed in a straightjacket.

"Why hello there!" Joker said as he spotted Harleen. "Are you the new doctor?"

"Yes," Harleen said in a professional tone. "I am Dr. Harleen Quinzel.

"Harleen Quinzel…Hahahaha!" Joker said with a sudden laugh. "If you tweak them about, your name would be Harley Quinn!"

"Yes, my friend said the same joke," Harleen said as she remembered how Danny commented on her name when they first met.

"So whaddya want to know about me?" Joker asked as he laid on the recliner. "My birthday? Favorite drinks? Hobbies? Color of underwear I'm wearing? What kind of underwear I'm wearing?"

Harleen sighed, wishing that she was Danny's doctor again.

For the entire hour, which felt so much longer, all Harleen could listen to was Joker's sick sense of humor. How his goal was to spread smile and laughs. This would have been an admirable goal, if it wasn't for the things she heard about Joker's laughing gas, a poisonous chemical that could kill people.

And from what she can gather, the man was obsessed with the Batman, claiming that the Bat's was the only one who could understand him.

She learned that he started out as a small town thief called the Red Hood, which Batman chased to a chemical factory. That chase made Red Hood fall down a vat of chemicals, which burned his skin white and ruined his psyche. Since then, the Joker was born and spread chaos wherever he goes.

Compared to Danny, Joker seems seriously messed up. Joker seemed to only care about causing chaos and mayhem. And she knew that he was beyond help.

"Well that was fun!" Joker said with a grin. "I can't wait for tomorrow! Though I don't think we'll have time tomorrow."

"What do you mean?" Harleen asked, confused.

"It's a surprise!" Joker said with a laugh.

Worried, Harleen quickly left the room as Joker kept laughing. As soon as she was out of sight, she quickly went to the Warden. When she saw him, Harleen immediately told what Joker said about tomorrow.

"I'm sure it's nothing to worry about Dr. Quinzel," the Warden said offhandedly.

"But he sounded so sure," Harleen said in a worried tone.

"He's insane," Warden said. "You're just too tired, so go home and relax."

"…Alright," she said reluctantly. Just as she was about to leave, she suddenly stopped at the door. "Um…How is Mr. Fenton?"

"A-13? He's fine," Warden said. "He seemed disappointed on having a different doctor, but he doesn't complain about it."

Harleen nodded before exiting the room. She was happy that Danny missed her, she missed him too. Conversations with him was better than studying the Joker's mind.

The Next Day…

It was chaos.

In the middle of the day, during lunch, the unthinkable happened: Arkham has been taken over by the inmates. It happened so suddenly that it took everyone by surprise. The Joker somehow managed to sabotage Arkham's security and opened all rooms and deactivated the inhibitor collars. The guards were easily overpowered and the inmates were running the building now.

Harleen was running as fast as her legs can carry her. Three inmates were behind her, all crazy. The Joker ordered the inmates to capture all the doctors, guards and the Warden, so that's why they were chasing her.

Harleen tripped as her high heels broke. She groaned before she saw that she was surrounded. With a roar, the three inmates pounced at her. She closed her eyes, waiting for the pain. Instead, she heard grunts of pain.

She opened her eyes and was shocked to see Danny in front of her, knocking down the three inmates. When he turned to look at her, Harleen could see the worry in his eyes.

"You okay Harley?" Danny said as he kneeled next to the blond. "I've been looking everywhere for you. This place has gone mad."

"Yes, it has," Harleen said as she stood up. "How did the Joker did this?"

"Who knows," Danny said before grabbing her hand and leading her through the hallway. "We better get you out of here."

"Me? What about you?" Harleen asked as she kept up with Danny's pace.

"Someone's gotta take this guys back in their cells," Danny said. Harleen suddenly stopped running, causing Danny to look at her.

"I won't leave you here alone," she said as she looked at Danny. "I'm staying here."

"It's too dangerous," Danny said. "I don't want you to get hurt."

"I won't leave you here alone," Harleen said with a determined look.

"…You're so stubborn," Danny said with a sigh. "Let's go and help the hostages first."

"They're in the Medical Bay," Harleen said as she remembered where the hostages were being taken.

"Then let's go," Danny said before the two ran to the direction of the Medical Bay.

Later…

"Wow, you're crazier than the rumors say," Danny said as he looked at the mysterious vigilante.

Said man just glared at him, leaving the inmates hanging on their feet. He was dressed like a giant bat. Harleen stared at the man with wide eyes as she knew who this man was.

"So, there anything I can do to help?" Danny asked, looking at the Batman.

"You can help by telling me where the Joker is," Batman said with a glare that didn't made Danny flinch.

Later…

"Let her go, Joker!" Danny ordered to the deranged lunatic, who was holding Harleen as a hostage. He was just foiled by Danny and Batman, but he didn't want to admit defeat. He sucker punched Batman to unconsciousness with a crowbar.

"Why should I, Freak?" Joker said as she held the knife closer to Harleen, who was struggling to free herself.

"Because if you won't, I'll hurt you, very badly," Danny threatened, his eyes turning green.

"Ha! Like you have the guts to have that kind of fun. You wouldn't even take down Croc after trying to eat you," Joker taunted before looking at Harleen. "You know, she won't look half bad with a Harley Quinn theme outfit. Don't you think so?"

"I'm going to ask you again," Danny said slowly. "Let her go, or else."

"Or what? You'll make me a crystal necklace?" Joker taunted. He didn't noticed Harleen before it was too late. "Agh!"

He held his elbowed gut as Harleen tried to get away. Joker managed to grab her by the arm and roughly pull her back. Before he could do anything, a green beam blasted him off his feet. He slammed on a wall and fell unconscious.

"I tried to warn you," Danny said to the Joker before rushing towards Harleen. "Are you okay, Harley?"

He was surprised when Harleen suddenly wrapped her arms around him, sobbing on shoulder. He patted her on the back, whispering soothing words.

A half hour later, Batman woke up and saw the Joker trapped in a block of ice. He then saw Danny and Harleen fixing the security system.

"It's about time you woke up," Danny commented as Batman stood up.

"What happened?" he asked in a serious tone.

A Week Later…

"Are you sure you want to quit?" the Warden asked to Harleen, who nodded.

"This job is too exciting for me," Harleen said, her hair now in a ponytail and was not wearing her white coat and glasses while wearing the crystal Danny gave her. She was also carrying a box full of her stuff. "Maybe being an Arkham doctor isn't for me."

"If you're sure," Warden said with a sigh. Harleen was one of the youngest genius this asylum ever had, so it was a shame to see her leave.

With that, Harleen walked out the asylum. She waited by the door until she heard it open. She turned and smiled as she saw Danny, who was carrying a knapsack and was wearing clean clothes. His hair was combed and his face was cleanly shaved.

Danny took a deep breath and exhaled. He gave Harleen a smile before the two kissed.

"Ready to go?" Harleen asked with a smile as the kiss ended.

"You bet," Danny said before the two walked towards her red convertible. "It feels good to be out in the open again. Thanks, Harley."

Harley gave him a grin before she drove away from Arkham, ready to start a new life, with her new boyfriend.

So there you go. I used the TAS version on how Harley met the Joker. I think the only reason she even fell in love with the Joker was she was really lonely and was manipulated. So I thought that if a less crazy and nicer inmate befriended her first, she'll fall in love with him. So I made Danny the nice and kinda crazy, talking to yourself does make you sound insane.

This also happened years after TUE, six to be exact. So a grief-filled Danny and a therapist working in an asylum seems like a perfect match, to me at least.

Also, I'm in a slump due to school work, so I'm gonna use the one-shots to try and get out of my slump. Hope you guys like this one-shot. Next cartoon girl will be Gwen Tennyson from Ben 10: Ultimate Alien! See you later!

Edit: I meant Alien Force. That was really embarrassing. Sorry about that!