Disclaimer: If I owned Hetalia, I'm pretty sure I wouldn't be the broke person I am.
Reader Caution: The following may contain triggering elements. This is the only warning for the entire story.
Chapter One: England
Arthur exhaled with a gentle smile as he ingested the pills. It might have been one too many, but surely he would be fine, he'd been taking these for several months. A faint tinkling sound from the corner of the living room caught his attention. "Ah, Flying Mint Bunny!" The green creature fluttered her wings, smiling happily. "Hi Arthur!" She flew a few circles around his head. "I'm so glad to see you, these last few days have been crazy." Arthur laughed happily, closing his eyes. He felt really tired... Maybe a quick nap.
"Come on, we can talk upstairs." With a tired smile, the Englishman climbed the stairs to his room. He leaned slightly on the wall as he got closer and closer to the door. It was just a couple more steps... Right? Why did the distance seem to be getting larger...? "Are you okay, Arthur?" Arthur smiled at his- not imaginary, thank you very much- friend. "Oh, don't worry Minty, I'm fine. Just a little tired." He borderline collapsed on to his bed, not even fully on it. The flying bunny landed on his bedside table. "Arthur! Are you sure you're okay?" Arthur shook his head feebly. What he had taken... It had obviously been an overdose.
"G-Goodbye, Minty..." Arthur found it more and more difficult to keep his eyes open. A heavy exhale, and they closed as he felt his consciousness drifting away, farther out of his grasp. One last, frantic inhale, and Arthur Kirkland's eyes would never open again.
It would take three missed calls from his older brothers, six weeks with no one seeing so much as a flash of blonde hair behind closed curtains before anyone realized what happened.
Arthur/England: Drug addict. Died of an overdose of hallucinogenic drugs.
This explains his 'Imaginary Friends.'