SOLVED. Fantasy book. Two main characters, both YA a boy and a girl. Book touches on death, the afterlife, the existence of God, different worlds/ dimensions. Read 2013-2019. Possibly blue cover with the boy on it. Spoilers. [s]

[Tony]

Let me preface this by apologising if this is somewhat vague or confusing,english is my second language and quite some time has passed since I read this book. I also was not sure about the genre and that's why I just put fantasy :(
I will put (?) next to the details Im not sure about.
As I said in the title the two main characters are a boy and a girl. I think that the boy is around 14-17 years old and the girl is 12-16 years old.
I am pretty sure that the boy has white skin colour while the girl has black skin colour.
The boy is living in a normal town. I believe he is living with his mother (?) who is introduced at the first couple of chapters but dies shortly after (?) possibly due to sickness.
The girl is living in another world/dimension. I think that in her dimension all the grownups got killed/disappeared cos in a later chapter she is playing with some kids of similar age in a lake/river and a point was made that they were all below 18.
One of those kids dissapeared while she was there because he turned too old (?)
During that chapter a point is made that the girl is extemely charming and able to befriend people fairly easily.
At a later chapter the girl is being chased by a large group of soldiers and a pilot (?) who was stuck at a dimension/place that was full of snow stays back to help her escape and dies while taking down either most or all of the soldiers, due to having the high ground. I think that the soldiers were dressed in white. The man died from bullet wounds and his weapon was a rifle or a bow.
The 2 main characters fall in love and kiss at least once.
At some point in the book it is discovered that every person has a reaper following them until it is their time to die and then he helps them cross the river to the afterlife. The reaper might be shackled to each person's foot (?). The reaper is invisible to most humans but for some reason either the girl or the boy could see them after some point in the book.
The reapers are depicted as kinda understanding beings.
The main character's one was pretty empathetic at times.
One of the main characters is able to talk to his/her reaper after some point in the book.
One of the main characters stays at the house of his/her reaper for some time and other reapers are there. (?)
One of the main characters is accompanied by their reaper into a river. I believe that crossing the river is supposed to kill the person. One of the main characters either goes in or completely crosses the river to save the other one (?) but doesn't die.
I believe that the boy was the son of an important being (?)
The boy had a dagger in his possesion which seemed pretty important during the whole story (?)
I believe that in the book there was an inner conflict between angels or angels and devils (?)
God was basically too old and weak to rule anymore and he was being basically safekept by some angels who slowly assumed power and ruled in his stead using his name. I believe that was one of the reasons that caused the conflict I mentioned. If I recall correctly God during the years of the main characters was incompetent perhaps even cruel and that caused chaos to the worlds/dimensions.
God's main protector was a male angel (?)
A world/dimension war broke out at some point because people wanted to kill the God (?)
the God eventually died when he left Heaven, if I recall correctly there was a fight between those accompanying him and the rebels (?)
I think that he was in some sort of protective sphere and he kinda melted to the air when that dissapeared (?)
I think that he was happy when he died cos he had lived past his time and just wanted to rest.
the girl and the boy were kinda close to the place where he died (?)
I think that the boy had something to do with his death but I'm not sure.
The girl had some sort of animal/pet that was extremely intelligent and protective of her (?)
The cover of the book was all blue (?)
The book was translated in Greek.
If you have bothered reading till this point I thank you from the bottom of my heart, I've been trying to find this book for ages.

[Sue]

I think you might be looking for the His Dark Materials trilogy, written by Philip Pullman.

[Sue]

Sorry, I forgot to add the links.
The Golden Compass
The Subtle Knife
The Amber Spyglass

[Tony]

Thank you so much Sue.The book I was looking for was "The Subtle Knife". I would never expect it to be found in the same day and the fact that it is part of a trilogy was such a pleasant suprise for me. Truly thank you from the bottom of my heart.