The prologue is written from Tails' point of view. The rest of the fanfiction is told in a more normal narrative. I don't know, I thought it would be an interesting writing choice. The whole "four year wait" thing was inspired by ThomasElias' "A Joyful Reunion".

"Well, guess I'll finish it tomorrow, then."
I packed up all my tools just after tightening one last bolt on my plane's exterior. I've been working on it for a week straight now – it's not like this upgrade was particularly important, or the circumstances asked for a better plane, it's just that… It helped me get my mind off of a particular person. A person who I fell in love with four years ago…Who had sacrificed her very life to save this world. Just as our relationship began to develop, it had to unexpectedly and abruptly come to an end. And I was the one who helped her fulfill her fatal destiny. She died by my hands… It's a long story, really, and I don't feel like recalling all of it.
Soon enough, I just got bored of work. Yeah, I guess her absence makes even my favourite field of work seem completely empty and devoid of any interest. And with that, I put my tools down and headed back into the main part of my house. Upstairs, in my bedroom, there was left a tiny reminder of that person – a little rose. All that was left of that person was nothing more than a small seed, that later grew into a rose that looked fairly ordinary at first…But somehow, this rose had greatly exceeded the lifespan of a normal one, as it was still alive and blooming. Not that this meant anything to me – having a false hope is worse than having no hope. That rose was a remnant of her flesh and blood, sure, but I doubt it would be enough to revive her. Two of my friends attempted to revive her after her sacrifice, but to no avail – the tiny white seed was all that was left of her. At least I can honor her memory…
As I went upstairs, taking each step rather lazily, I slowly began recalling all the memories I had about that girl. As much as I tried to think about the more pleasant ones, I just could not – all the memories I remembered the most, unfortunately, were two unpleasant moments of our lives.
One was when a certain person – ironically, he was one of the two people who tried to revive her later – had attempted to kill her. We were clearly outmatched, and despite our best efforts, he managed to catch up to us anyway and had attempted to finish her off…Only for someone to intervene at the last possible moment, and that's when I learned the painful truth about her being a sleeper agent for our enemies. The pain of her unwilling betrayal was almost nonexistent – I couldn't care less even if she turned out to be secretly working for them on her own volition. What really pained me is hearing her scream from the pain of realization of her unwilling betrayal. It took me some effort to reassure her later on that we will do everything in our power to defeat those who made her an unwilling spy, and that we would still protect her no matter what. And the other unpleasant memory was…The day of her sacrifice. I tried my best to repress that memory. I tried my best to move on. I tried my best to just forget about all that happened…But such memories don't just fade away. And this is when I understood the pain of unwilling betrayal myself – I had to practically kill her off by my own hands so that her sacrifice would be complete. The cold feeling of sorrow I had when I saw her practically get disintegrated together with that damned monster she tried to weaken at the cost of her own life… The false hope I had when I noticed that someone actually attempted to revive her… And the crushing breakdown I had experienced when I saw that their attempt was unsuccessful.
While I was reminiscing of those painful memories, I almost didn't notice how I was already standing in front of my bedroom door. I sluggishly opened the door, a very slight smile crossing my lips - getting to see the only remnant of her that was left in the mortal world was the only thing that made me feel like there was something worth living for. Unfortunately...It seemed that even that was taken away from me - the plant was absent from the window shelf.
"No…" I immediately looked around under the window in desperation. No pieces of a broken pot, no ground scattered across the floor. I was so busy with the small details that I didn't notice the elephant in the room - the window was open for some strange reason. I immediately rushed downstairs and out of the house. If the plant fell out of the pot, I'm pretty sure it can still be saved. Just got to make it in time… As soon as I approached the area just slightly under the window...
I saw her again – lying there, leaning against the wall of the house, still having that green and white dress on her that she always wore, completely unchanged. In fact, she even had her red crystalline oval-shaped amulet attached to her dress - the same one that she used to set her sacrifice in motion. She was still much of the same. However, it did seem like she aged through these four years that had passed, as she was a bit taller and her body was more developed, her green leaf-like hair was long enough to reach just barely below her shoulders, and the twin red buds on the sides of her head were on the verge of blooming.
I couldn't believe my eyes. Was that another hallucination I had? Did her death really hit my psyche so hard that I began deluding myself like this?I rubbed my eyes with my hands, closing them and reopening them again – she was still there, lying under the window silently. I ran over to her and ran my hand across her hair – as obvious as that may sound, I actually felt it. I hardly ever got to touch her, and only once did I ever touch her hair back then, but I still remembered it all too well, and it certainly felt the same as before… She was breathing quietly, so I figured she was probably just unconscious. I breathed a sigh of relief at that, but then I thought I might as well take her back home. Making my way upstairs and gently placing her in the bed, I sat down next to her. Soon enough, it seemed like she was close to waking up.
"Are you alright, Cosmo?" – I quietly asked to see if she would respond. She turned her head side to side for a bit, and then slowly opened her eyes. I then realized that I could have made a mistake here – what if she didn't actually remember me? Then she would probably be very surprised to hear me calling her some seemingly random name. Or to see some random orange twin-tailed fox staring at her in complete disbelief…
Thankfully, my suspicions weren't true – as she made out who was in front of her, she suddenly jolted forward, embracing me and shouting: "Oh, Tails! I'm so glad to be back with you again!"
I couldn't believe what I heard just now. She clearly remembered everything. I myself couldn't resist hugging her, and as much as I wanted to confess her all my pain, I was feeling a bit too tired, so I just quietly whispered: "Welcome back, Cosmo. I really missed you." – I didn't question her on how she was able to return, or anything like that – it didn't really matter to me at the moment.
"I…I was probably gone for quite a long time, wasn't I?" – Cosmo asked, letting go of me. With slight sadness in my voice, I reply: "It's been four years since your disappearance."
And there she was, sitting on the bed completely silent for a few moments, with a look of utter disbelief on her beautiful face. I guess that's understandable - she had no way of contact with the mortal world, and even if she did, she likely lost track of time.
"Are…Are you okay?" – I ask, sounding quite concerned.
"It's nothing, really. You probably found someone else while I was gone, though..." - Cosmo said, looking down in dismay. I embrace her once more, and reassure her, replying in a determined voice: "No. No, I did not find anyone else while you were away. And I didn't want to, anyway - you are irreplacable, Cosmo, as you are the only person I truly loved. And will still love. "
At first, she looks at me, frowning confusedly, but soon the frown fades from her expression, as she smiles and replies: "I love you too, Tails. Sorry I didn't get to confess it on more...Peaceful terms. " - And as she says that, she leans in to kiss me on the lips. That was it. That was our real first kiss. Sure, back then, after the tragedy, there was...A certain vision I had. I saw what I assumed to be Cosmo's spirit appear before me and seemingly attempt to kiss me on the lips. But of course, her being a spirit, she just passed through me and faded. And now, I felt it all for real. There was no doubting it. But all good things have to end, and I soon let go of her, needing to catch my breath. "I'm glad that you had returned." - I quietly reply moments later, looking into her beautiful sky blue eyes.
"And I'm glad to be with you again."
And with that said, we quietly look each other in the eyes for a few moments before she looks around and asks: "So, where exactly are we? And is it just me, or it's a bit too dark here?"
"We're over at my house, in my bedroom. And it is late evening on this planet already, so of course it is dark here."
"Oh…Right, that was quite silly of me. So, are you going to tell the others about my return now?" – The plantgirl asked.
"Not yet. I guess it's pretty obvious that they're all likely back at their homes, and are probably sleeping. For now, how about I show you the rest of the house?" – I replied as I helped her stand up and we went downstairs, into the living room. She looked around for a bit, getting acquainted with her surroundings.
"I…Kind of expected your house to be a bit more technologically advanced. Not that I'm disappointed or anything, but…" – Oh, the same defensive tone she always uses when she feels like she said something wrong… I chuckle and reply: "Well, I'm sure the security system here should probably match up to your expectations. Then again, there isn't anyone who would want to break into my house anyway."
And so, for the time being, I was just quietly showing her around the house, telling which room is for what, lastly showing her to the guest bedroom, as it was where she would probably be sleeping from now on – I doubt anyone else would offer her a place to stay at.
"If the bed doesn't seem comfortable enough, then I'm sorry about that. It's just that I rarely have guests, so I didn't bother putting a better bed here…" – Just as I kept speaking, Cosmo quietly sat down on her new bed, even feeling it up with her hands.
"Seems soft enough. At least softer than the bed I had in my room on the Blue Typhoon." – As she said that, she yawned, covering her mouth with her right hand. – "I'm already feeling quite tired, to be honest. I'd like to talk to you some more, but even though I was kind of, you know, 'asleep' for four years, I still feel sleepy."
"Well…Even though I have quite a lot of things that I should have said way earlier, I do agree that it's already getting late. We might as well go to sleep now." – As I said that, Cosmo took off her green shoes and lied down on the bed in the guest room. As the weather was still quite warm, she didn't cover up with the blanket, just lying down on the bed and closing her eyes. Still facing me, as I stood at the side of the bed, she quietly murmured: "Good night, Tails. I'm glad to be in the mortal world again. And I'm especially glad to be together with you as well. "
"I'm also glad you're back with me. Sweet dreams." – With that said, I kiss her on the cheek and go to exit the room. As I return back to my bedroom, I take my gloves and shoes off, jumping down into my bed.
"Never had I thought that today would be the day when I met her again and had a chance to confess everything to her all over again. Am I dreaming?" – I pulled a small strand of my fur out from my hand and yelped from the slight pain. – "No, I certainly felt everything for real. Damn it, Miles, since when did you develop paranoia? Just lie down, accept things the way they are, and go to sleep."
Soon enough, my mind was almost completely fogged out from how sleepy I felt. The pin-drop silence of the room and the strangely sweet scent of the night air – I had slightly opened the window to make sure the air here doesn't get completely stale overnight. – didn't help on the matter either, and I soon fell asleep.

I wonder if anyone's going to notice the fixed-up prologue?