Author's note: to answer your question, Katara, Cake and Lord Monochromicorn's relationship really isn't going to come up in this story. But if it did, then yes, they would be dating, because they're adorable. :)

It really wasn't that hard to find Fionna and Ignus. It was a perfectly clear night, and it was no secret that Fionna loved looking at the stars. We only had to check a couple good stargazing spots before we found them.

They sat close together, holding hands through tin foil and oven mitts. Ignus's fire lit the area in a soft glow that danced across the shadows. It was so horrendously cute I almost felt like puking. Any normal onlooker would probably be awwwed by the so-called 'adorable' couple, but seeing him with her like that made me sick. How dare he sit with her like that after hurting her like he had. I crouched in the bushes, Gumball at my side, ready to jump out at any moment.

I couldn't quite make out their conversation, but from their voice tones it seemed calm enough.
"See?" Gumball whispered. "Just look at them. There's nothing to worry about."

"I'm not so sure. This guy's always given me a bad feeling. Let's stay a wile, see how things play out."

Gumball nodded and returned his attention to Fionna and Ignus.
"All right, Marshall Lee, you win. We'll stay. But I'm telling you, you're getting yourself all worked up over nothing."

We sat there for quite a while. Long enough for Gumball to lose interest, anyway. He was sitting against a nearby tree scribbling chemical equations in the dirt. I, however, was just as poised and attentive as I had been all night. I still couldn't quite make out their conversation, even with my vampire hearing. But Gumball and I were dangerously close to the edge of the clearing as it was, and getting any closer would surely compromise our cover. I could pick out a few words and phrases though, and before long I caught a piece that snagged my attention.

"What...Marshall...Gumball?" It was Ignus speaking.

I gestured to Gumball, "Dude, they're talking about us" I whispered.

"...seem very close..."

"Well I never-" Gumball muttered before I hushed him.
"Nah...bros..." It was Fionna's voice this time.
"Well...an awful lot...'just bros'" Was it just me or was Ignus burning a bit brighter all of a sudden?
"...Those guys...best friends!"

Gumball and I didn't have any trouble hearing the next part of the conversation.
"What if that's not what THEY think? Hm? They think I'm dangerous! They're trying to take you away from me!"

"No, Ignus, that's not-!"
"DON'T LIE TO ME!"

Flames exploded from Ignus's body. Fionna jumped back just in time.
"Ignus, please, calm down," Fionna pleaded with him.
"They won't take you from me! You're MINE!" he shouted as he grabbed her arm. A sizzling sound could be heard, and Fionna screamed.

The perks of being a vampire. It took me only an instant to dart from behind the bushes to the center of the clearing. I grabbed Ignus's wrist, twisting it and forcing him to release Fionna. Fionna fell backwards into Gumball's arms.
"Are you alright?" he asked her.
"I...I think so..." she replied.

I threw Ignus back, and he glared at me.
"I knew it! You're trying to take her from me! Well you can't have her! She's MINE!" Ignus roared.

"No. She's not." I stated. I was at that level of anger where you're strangely completely calm. "Fionna isn't some posession. She belongs to no one but herself. And either way, you don't deserve her. You'll never touch her again. Not if I have anything to say about it." I stood protectively between Ignus and Fionna. Glob help him if he tried to move me.

Ignus's flame burned hot and bright as I've never seen it before.

"So that's how it's going to be, hm?" He laughed a sinister laugh. "Well then, I'll just have to get rid of you!"

He charged at me, swinging his flaming fireball of a fist straight at my chest. But he didn't stand a chance against me now. As I reached out and grabbed his fist, stopping him in his tracks, his expression completely changed. I heard Gumball gasp behind me. Ignus stared at me with the most horrified look on his face.

"What...what are you?" he stuttered.

"Your worst nightmare."

If anyone's wondering...this is how Igunus's and Fionna's conversation actually went:
"What's going on with Marshall Lee and Prince Gumball? You seem very close with them."
"Nah, me and those guys are just bros."
"Well, you hang out with them an awful lot to be 'just bros.'"
"Well yeah! Those guys are like my best friends."