Warning: ghost kids or something idk

An old man in a wheelchair gathered all the souls of dead children he could get his hands on.

They became his personal army. The children were all given a uniform based on the kind of combat they specialized in (melee, gun, machine gun, sniper, scout) and a weapon that best fit them.

The protagonist is a 15 yro girl, blonde, freckles, with two short pigtails on the sides of her head. She is a gunner.

Her friend, male, slightly older, was a melee-er with stun batons.

The old man sought to gain his youth back, to do so he used the energy of the ghost children cores. He was tricky, if you accepted to be part of his army he will protect you and you won't go to the underworld, but in return he will take your core. 'It won't hurt, not for much anyway. Then you just have to do as I say and you'll live like a prince/princess'

All the kids that accepted the offer had a hole in their diaphragm bordered with many small pink crystals.

He gathered thousands of children, but the energy is still not enough to grant him his youth back.

There is a guardian of sorts, he's called the Big Wolf. He has a human-like appearance mixed with wolf ones. His fur is completely gray. He also has a few scars (a bite on his chest, one on his neck and a few gun shot scars all around. He's dressed in a silver light armor.

He is part of the underworld, but he's the only guardian it has. So he fights alone. He knows everything regarding the dead kids and the underworld, the old man seeks to capture him so he could get the info he needs to fulfill his task, but the Wolf is an extremely good fighter and he can escape any kind of cage he's put into.

The Big Wolf never talks, but yelps or whines when he's hurt. He understands human language completely, he can read, write, listen and react to what he's told like a human would.

The protagonist isn't part of the army, but sometimes has to blend in and help her enemies so things won't get out of hand. At some point she has to blend in so much she has to kill other people or she will get killed.