He is 8 years old here

Valt: Mom, mom!
Srecan runs to the kitchen
Mom: Yes Valte?
Valt: Let's eat! and they pull her by the hand
Mom: Wait... take it easy and laugh
Valt: And you, dad, eat with us
Never!!!
Toko: Yes!!!
And everyone laughed
In the Aoi family, there was always laughter and joy...never sadness...
But one day...

Valt pov: I'm coming back from school and I can't wait to see what mom made for lunch, I'm very hungry!

I entered the house and saw some people...and I heard Toka's mom and Nika crying
Valt: Mom!?
Someone took me...it was my dad
Dad: Everything will be fine Walt... calm down
Valt: Why are you all crying? And who are these people!?
And they just took my family away
Valt: NO!!!!!! LET THEM GO!

He cried and screamed
Some man wanted to kill him...but someone hit him on the head and he passed out
Valt: Thanks...and who are you?
The man laughed: Do you want to take revenge on those people who took your family?
Valt: Yes...

And since then,Valt learned from that man... until he was killed by the same people who took Valt's family away (They didn't kill them... because they wanted to catch Valt as well)

Present:
Valt couldn't forgive himself, but it wasn't his fault...
He wasn't there even when he came and saw the man who saved him dead... from then on he became very cold...

And now he is 15 years old
He is slowly killing those people who took away his family
He looks for them... and kills them,and he also has a task...
He needs to protect a girl...that's what his teacher told him before he was killed...
It was Y/N...the best singer in Japan, but Valt's assistant gave him the wrong information and Valt was given a new task to kill her...
Her or another girl?
And is that Valt's real helper?