Using pleinair in the beginning feels like cheating to me

[Defused Bomb]

She's just so much stronger than everything else you start with in the beginning of the game and I feel like the game starts off way too easy with her on your side. Does her strength fall off relatively quickly cause I was thinking about going through the game without using her just to offset the imbalance.

[Lipucd]

It was the same thing on the DS version. You get her rather dang easy if you lose towarsds Mid-Boss and then on the New-Game+, got her right out the door. She's very strong for the first few areas, and helps out a ton of you're trying to do Item Worlds that early in as well. She does start to even out though around the end of Etna's main story plot points, and by the time your done with the 'base' game and working on the 'end' game stuf, she's about as good as any normal gunner unit.

[Defused Bomb]

I got her at the start of my first run of he game :o is that normal?

[ShinkuTear]

We aren't 100% sure how she is supposed to unlock in this version, she might be meant to join right away.

She is a bit over powered at first, yes. However, she also falls off later. The 'ultimate' generic units all have at least 100% aptitude in every stat, and fairly high base stats. To compete with them, Pleinair might need Reincarnation, and to take advantage of her high SPD aptitude. That 80% in DEF is pretty major.

[Son of Riggnarok]

You think she's OP? Just wait till you get a Majin.

[PFC Demigo [29th ID]]

She can be recruited by selecting the blank option in the Dark Assembly during a New Game+. She has the class called Mascot. Her unique skill is Usagi Drop, which drops a few copies of Usagi on an opponent. She has high aptitudes for SPD (being the only class other than the Ninja to be able to dodge special attacks and spells) and Hit, making her suited for using guns

What makes Pleinair so good is that unlike other characters, the only stats she needs are Hit and Spd.
Her godly speed aptitude, combined with the ability to dodge Magic and Skills mean NOTHING will hit her (Unlike ninjas though, she won't dodge healing spells. Yay!).
As for attack, giving her a gun means she has ranged attacks, and Hit doubles as both her attack power and accuracy.
The biggest weakness of gun users is that they can only target single enemies. Usagi Drop lets Pleinair hit multiple enemies, and if she has a gun equipped, its power is based off her Hit as well.

At the point in the game you can get her she's very powerful. She has 150% aptitude in Speed and can dodge any attack including special attacks and magic, so if you level her up and keep her speed high nothing in the game will ever hit her.
She also has 120% aptitude in Hit so she's good with guns (although she can use any weapon you want really, she gets an A or B rating with all weapons except bows).

Basically she is a Game Breaker.

[Quantum Optimist]

yeah i noticed how crazy good she is. She isn't even wearing any armor, only glasses, hp thingie and another thing i don't remember and she gets more hp than my main guy with all hp increase i could get, she hits twice as hard as him (one shot everything at max range) and almost never get hit by anything even if they are 10 ennemies surrounding her! I can send her alone to tank multiple mobs and she barely get scratched!

A bit unfair to all the other fully tank equiped warriors that can't tank as well as she does!

[Lunk]

So I would just take her as a little gift to help you get started with the real fun of Disgaea, which is powerleveling. She get's you to the first viable powerleveling stage, then she's just another normal character.