Childhood:
While PunPun is small, he is drawn as a simple bird without details; he is white because he does not know anything yet. He is white because it reflects his knowledge about the world and adults. His innocence and his ignorance about the world. PunPun is a bird but has no wings, he thinks he can fly over the world and conquer new planets, he is still a child.
(foreshadowing to when he tries flying with umbrellas)
Scuole medie:
In questo periodo è separato da Aiko e non sa più cosa fare. Il suo passato lo rincorre e incominciano le prime crisi, diventa avido e il suo desiderio di sesso diventa sempre di più. Impara sempre di più sulla realtà e inizia il suo escapismo. Qui incomincia a soffrire
Freedom/Umbrellas/Maturity
PunPun here wants to run away because he feels completely lost. He wants to grow up and become an adult because he thinks that by becoming an adult you learn more about the world. (Understanding why his father lies to them, understanding how to relate to others, etc.). PunPun wants the freedom of being able to become an adult and learn everything quickly. PunPun wants white wings however, he is represented flying with an umbrella. The fact that umbrellas give the ability to fly is just a fiction invented for children, a mere illusion, PunPun is not mature enough to become an adult. Exactly becoming an adult does not automatically mean becoming free nor does it mean understanding/knowing more. The reality of the situation is something PunPun refuses to acknowledge. Precisely at the end of the sequence we see him depicted lying on the ground.

Selflessness and growth: chapter 43
PunPun through his experiences learns more about the world and realizes that Yaguchi would be a better companion than he is, he is no longer a child with an umbrella trying to fly too high but noting reality.

PunPun triangle: ch 77
PunPun is in a bad period, he hates a little bit of everything and everyone, he misses Aiko, and he is in pain, so the triangle shape without legs and arms represents how he cannot move forward, he is stuck in a trap of his own related to the past. he has no arms or legs, it shows how he cannot move forward.
PunPun cannot move forward. He is stuck in his past. But when he meets Sachi his limbs begin to be drawn again, representing how he is attaching himself to Sachi in order to move forward.

The moment he gets drunk and thinks back to the past again (Aiko) we jump back in time after he has fucked Sachi, and is again represented without limbs, he falls back into his mistakes again

After his sin PunPun in Sachi's presence is depicted with his hands visible, the hands that did the sin. In contrast, his legs are still not visible, since legs would give him the ability to move on and leave behind his sins, but he cannot at that time. (Of course, this is all metaphorical)

Death/Adult
PunPun is a bird, and although he has grown up he has not yet developed wings, this is because he keeps clinging to something in order not to face reality. In doing so he ends up dying.

Adult PunPun: ch 100
Once reborn/grown PunPun denies his past and tries to escape from it, he changes his personality, the way he behaves and even his name. He changes his identity. However, it is not enough precisely he is not happy
PunPun adult black: cap 102
PunPun adult black: cap 102
In this form he has 4 eyes, and they are no longer just dots as before, now they are well-drawn eyes. PunPun now has real eyes that allow him to look reality in the face. Once he sees reality he doesn't believe it, he doesn't even believe in its appearance. Exactly shortly after that he becomes violent

Adult horns
In his last phase PunPun disavows all that he has become, no longer loves, no longer suffers, becomes indifferent toward Aiko. He becomes indifferent toward himself.
Embrace/God
Aiko holds on to PunPun with both arms, she cannot let go of him, she only holds herself together because of him, she cannot stand alone. She is completely dependent on him.
PunPun holds on to himself (The 2 hands) because he doesn't hold on to anything, he no longer needs a means of escape as with Aiko, now he is his own god.

Shape Imbalance/Desire
Just before the attempted murder PunPun returns to his basic form, this is because he is again feeling something "I'm glad I met you"

Shortly afterwards PunPun throttles Aiko and here he returns to his black form, this is because he does not have to prove anything more; by eliminating Aiko he could become free.

PunPun/God
In the beginning of the story God is a means of solving PunPun's problems. But he never solves them, giving only vague answers, which makes sense later, since God is PunPun's subconscious. Being an integral part of PunPun he cannot give him answers that he himself does not know.
Later we understand that God is nothing but PunPun's evil part, a part of his evil subconscious. He is the representation of PunPun's pessimism and negativity.

Precisely after PunPun's 20, during the final arc we often see God in PunPun's eye. God is in the eye because he knows everything PunPun sees, being a part of him. every action of God is simply guided by PunPun, they are not 2 separate entities

PunPun once Aiko is dead can live as peacefully as he always wanted, and to do so he must eliminate god, he must eliminate a part of himself, and PunPun by scratching his eye metaphorically kills the dark part that guided him.

PunPun/Aiko
Their dynamic is structured in such a way that PunPun and Aiko mirror each other in opposite ways. PunPun in his childhood years into his 20s clings to Aiko, and trying to keep up, trying to catch up with her sinks. Aiko makes PunPun suffer. After PunPun's 20 the situation reverses, Aiko clings to PunPun and he moves on, here it is Aiko who suffers the most. She is abused by PunPun and her life begins to revolve around him. During this time the 2 witness shooting stars again, this time PunPun is no longer amazed by them, while Aiko is, she hopes the stars will grant her a wish, she lives in an illusion to escape. PunPun, on the other hand, is not so cheerful and seems to understand their situation better.

PunPun/Sachi
They are the opposite of each other, PunPun is in love with Sachi (An attainable love) but still wants to go with Aiko (Unattainable love). PunPun keeps running away. Sachi is the exact opposite of PunPun. She is a person who embraces reality and improves herself. Sachi was criticized and that's why she made an effort to change, instead PunPun makes excuses and runs away. Sachi criticizes PunPun to improve him, and it is because of this that PunPun is saved, without Sachi's presence PunPun would have ended up like Aiko.

Human
PunPun's human form is seen for the first time and is deprived of God, now PunPun can live peacefully without having to chase anyone.
Vederlo umano non è chissà che cosa, la sua eliminazione di Dio e di Aiko lo avevano già portato a quel punto. La cosa magnifica di questa scena è il fatto che non completi la sua frase, PunPun non può più essere considerato Onodera PunPun /Fujikawa TakashI. Ormai lui è una nuova persona.

Final:
PunPun says goodbye to his current, human body. Symbolizing how he is leaving everything behind. He is now free from God

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I tranlated this from italian, maybe there's grammatical error.
PunPun,
The story of a young boy with a dream of escaping Earth and looking outward. The story of a boy struggling with life, trying to find the strength to move forward. The story of a boy searching for hope, searching for purpose, searching for meaning to his life, even when everything around him is falling apart. The story of a boy who could not find happiness and who did not want to find happiness despite the numerous times life allowed him to do so.
I suggest this manga, but you have to hold on, the beginning is nice, then it gets very nice, but it gets boring too many times. It becomes a really fascinating story only after chapter 100