Here's one: have the original Night Master show up and attack them by surprise...

Aside from those flaws that B2 had spoke of, this looks promising...

Your first? Well, if not for the however many miles of wire separating us, I'd clap you on the back and stamp your wrist with 'Welcome to the club'. But, alas.

Alright, let's get right down to the basics: I could tell this was from a new author. Nothing against you, as these things I can vaguely remember doing (I think, now, getting through this stage of fanfiction writing might be an unspoken rite-of-passage. Like a society of warriors tossing their youngest potential-soldiers out into the wild for a year). So, I think a condensed list of these trips would be best.

Avoid bolding your author's notes, as—especially in short chapters such as yours—this draws attention away from your material. Caps lock, to a lesser extent, does this as well, and both of these would best be avoided in the narrative, too. Most times. Caps lock has a tendency to look silly in third-person writing. Bold what you want the absolute most emphasis on, like a young girl, becoming so consumed with rage over the murders of her parents, swearing to track down the guilty S-O-B and do the same.

For author notes, I suggest using italics or underlines, or bunching it together with the summary re-paste and other such information.

Title: By that I mean this.
Genre: Criticism
Warnings/Tags: Character death
Pairing(s): YinxYuck and YangxLena (I'm guessing, because from the 'an old friend and an old enemy'-type dating pool, these two are the default)
Summary: And so on.

It's tidy, and easier for posting your story to multiple sites, in case you ever decide to do that.

Don't put author notes in the middle of your text. Again, it draws a reader out of the action, and is the biggest indicator of a new writer. If you want to add footnotes to a fanfiction, use little stars (*), or parenthetical numbering (1)(2)(3)(...). Keep in mind, though, that footnotes are more for memoirs.

Tell-and-show

/"Yin came down the stairs. She no longer wore that old woofoo training shirt. As the years passed and changed so did she. She was still very smart and brave and was no longer a woofoo warrior in training she was now a fool level 4 warrior, and with that came some new powers. Like the power of flight and water bending."/

The rule of 'show, don't tell' doesn't mean you can't have exposition, but that overuse of it is the sign of a poor writer. Instead of saying 'Yin came down the stairs', expand, add some scenery detail. Describe how her banging footsteps echo in their small little apartment (where -are- they living, anyway?) (are they working, are they renting, are they bumming around from friend's couch to friend's couch?).

Instead of "The news reporter on the TV commented on his attack on the town," show the actual dialogue. Since Carl is a b-list villain, he'd probably need to be with Herman to warrant any screen-time. Likely, the reporter would be overlooking him, leading to an argument between the reporter and Carl, likely ending in a zap to the seat of someone's trousers.

Instead of merely tacking on that she doesn't wear her gi anymore (yeah, technically speaking that's the name of the shirts they wear, one half of a gi), yes describe her current clothing, but slip the exposition in more gracefully. Have a scene where she's going through the closet and finds the box where their old clothes preside. Have her be slightly delayed in coming down the stairs by pulling on her shirt, catching herself in the mirror, and having a moment of disconnect between the young woman standing there and the tween rabbit who ran around in the same outfit everyday.

Expanding on the description and emotion is a great way to lengthen your chapters out.

Dialogue is an excellent place for exposition. So, you don't like Jobeaux, so you won't include him. Well, alright, but don't ignore his existence within the narrative itself. When Yin and Yang finally have their reunion with the gang, have one of them bring it up, and someone offer an explanation. (He has been crowned king of Redneckistan and must stay to rule; he has court appearances to attend in his divorce proceedings; whatever.) Have Yin or Yang, somehow, need to reiterate the details surrounding their father's death, coating their lines with emotion like coating sauce on a chicken.

But don't give up! I'm actually already impressed with/slightly envious of you—my language classes don't start for another year. It can't be easy tackling a language that says sentences like 'Will Smith will smith.' are 100% accurate.

Okay, I swear I was going to keep this short. I'll save the plot-commentary for later, but I will leave you with this: beta readers. I'm not the best to be giving grammatical advice; my friend Daedalus370, however, is. They can give you good pointers on grammar, spelling, and other things concerning writing style. Currently accepting beta requests too, I believe.

And with that, I say farewell, and happy writing!