Note: For the sake of clarity, whenever a character is in their transformed state, their name will be given in italics.
For example:
Helga G. Pataki = Helga
Sailor Moon = Helga
It was early evening. Arnold was in his room, sitting at his computer desk and doing his homework. He was just about to ponder the third problem of his math assignment, when he detected the faint sound of tapping from up above.
Arnold tensed. He was aware of the recent string of burglaries happening within the city—one had taken place right in his neighborhood—but so far none of the victims had ever confronted the culprit. Maybe the burglar was getting more daring?
Whatever the case, Arnold wasn't about to take any chances. He stealthily grabbed the baseball bat leaning against the side of his computer desk, then scaled the foothold on his angled wall leading to the windowed section of his ceiling.
The tapping persisted, and he could scarcely make out the outline of a body in the dark on the other side. Gripping the bat tightly, Arnold pushed open the window; almost immediately, someone had jumped through the opening straight into his room. This caught Arnold off-guard, and he lost his footing and fell to the ground below.
Arnold groaned as he lay on his back in the center of the floor. Slowly he opened his eyes. The first thing he saw was a young girl staring back at him, her blonde pigtailed hair so long that it was actually touching his face as she looked down at him.
The girl shook her head and smirked. "Swept off your feet again, huh? We've got to stop meeting like this."
---
Arnold and Helga sat together on the roof of Sunset Arms. It was an exceptionally clear night, and a few stars dotted the sky to keep the full moon company.
Arnold looked to his companion. "It's been a while," he said.
Helga nodded in agreement. "I know. Too long."
"I've missed you, Sera."
For a brief second, Helga was confused that he called her by that name, then she remembered that was the alias she had given him when they had first met two years prior. She had rescued him from a gelatinous monster and brought him back home in the hopes that she could uncover his secret identity. Perhaps this time, thought Helga, her endeavor would prove more fruitful...
"So what have you been up to?" Helga snapped back to reality when she realized she felt Arnold's inquiring eyes on her. She looked away tentatively.
"Oh...you know..." she mused. "I've been around..."
"I haven't seen you in over a year," replied Arnold, who then faintly smiled. "Are you still on a mission to 'protect the planet'?"
It was now Helga's turn to smile. "Well, it is a full-time job..." she turned to look at Arnold. "You humans are such droll creatures."
Arnold laughed. "No argument there." He then sobered a bit and looked up toward the sky. "What's it like to do what you do? Are you ever scared?"
"Nah, I'm not afraid. I've got a pretty good team behind me," Helga was surprised to hear herself admit. She was almost afraid that Arnold would question her on the existence of even more supernatural girls roaming the city, but he was already on his next thought.
"Are you ever worried about getting...hurt?"
Helga took her time to answer. It had crossed her mind, especially now since the creatures they encountered seemed more aggressive than the ones in the past. But she couldn't tell Arnold that – no need to unnecessarily worry him. She waved her hand nonchalantly. "Nah. This is nothing."

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Sailor Helga 🌙 | A Hey Arnold! / Sailormoon Crossover Fanfic
Fanfiction*#1 IN #HELGAXARNOLD TAG FOR APRIL 2021* Helga, Phoebe, and Rhonda meet with Luna the cat, who gives them powers to become Sailor Scouts. Also, why does that masked football-headed kid look familiar...? Later on, Lila, Big Patty and Diana the cat jo...