(This is a fanfiction where Sonic is nowhere to be seen, so he won't even be in this. If you don't like it, I can rewrite it.)
At Castle Acorn, there was (Your name), a (favorite animal) that can fight, but couldn't fight because he doesn't know WHO or WHAT he's fighting for. As for why he's at Castle Acorn is because he was sent by King Acorn to talk about something that will change (Y/N)'s life forever.
King Acorn: Ah, you must be (Y/N), I heard so much about you.
(Y/N): It's nice to meet you, your majesty, but why did you sent me, and how do you know about me?
King Acorn: It's because you're special, you have a legendary weapon with you, I heard you battled a few badniks before.
(Y/N): Oh, well, what is your purpose on sending me here for?
King Acorn: You're here to become a guardian for Mobotropolis.
(Y/N): But isn't Sonic...?
King Acorn: That's exactly the point, Sonic is missing.
(Y/N): What?! What do you mean?
King Acorn: Something happened after the battle between Sonic and Eggman, and Sonic was never seen again. We don't know what happened to Sonic, but we need someone to protect Mobotropolis. I know there are the Freedom Fighters, but after Sonic's disappearance, the Freedom Fighters were captured, alongside my daughter, Sally.
(Y/N): So you want me to save the Freedom Fighters and become Mobotropolis' new guardian?
King Acorn: Yes. Do you accept?
(Y/N):Hmm...
King Acorn: Well?
(Y/N): Yes
Sonic's missing? The Freedom Fighters are captured? This is what (Y/N) is here for, to become the new hero against Eggman. Find out what happens next time.