To immediately dive in: Childe is the poster child for C-PTSD. C-PTSD forms from experiencing repeated trauma for an extended period of time, the brain being forced to function solely on survival on a day to day basis, fitting Childe exactly, and is probably why I've, sadly, boasted him as one of my highest kins despite him being from Genshit (I am AR55 I can say that). Childe's trauma and his various methodologies of dealing with it and living with it are shown throughout his story and, your honor, make him So Me.
To begin: what caused Childe to form C-PTSD? Childe's trauma is, obviously, rooted from his fall into the Abyss. Although the experience isn't fully described in lore, it's easy to assume he had to be on constant alert, ready to fight at any given moment. It's here he met his teacher, Skirk, that allowed him to have the fighting skills necessary to survive in the Abyss and excel as a fighter in Teyvat. Obviously, having to be taught means he wasn't always a skilled fighter, and he undoubtedly would've had several close encounters during his time in the Abyss, on top of fighting requiring excessive alertness, especially when constantly in a hostile environment. Thus, being alert becomes a constant in his life.
Biologically, what is happening is stress hormones are constantly being pumped into the system, inducing the body into a constant state of fight, flight, or freeze. This state of mind is one that will continue to plague the mind and body for an extended period of time, and will affect relationships and viewing oneself moving forward. We can potentially see this in him being unable to form any meaningful relation with his peers–although such can also be blamed due to the eccentricness of the Harbingers themselves, who are… characters, to say the very least. Either way, there is a disconnect shown between them, with Harbingers either viewing their youngest as an oddity or with Childe looking upon them with disdain for how they follow through in their ways. Straightforward, Childe fights, and dislikes stopping to scheme like he peers frequently do.
Another addition to Childe's relations–although more of a headcanon–is a strained relationship with his family (younger siblings disincluded). Coming back to normalcy, Ajax (who had undoubtedly changed in an irreparable manner) is headcanon to have been ostracized from his family–those old enough to have seen the change. Ajax was once a frightened, hesitant boy, and within the span of three days the young boy had turned into a bloodthirsty weapon, quite literally losing the spark in his eye. He was unable to go back to living a normal life, and due to it, had his relationship with his parents completely uprooted. Unable to handle this new personality, the complete opposite of the young teen that walked off just three days prior, they sent him off to the Fatui, where his survivability made him thrive and Tartaglia was rightly born.
Upon the topic of Tartaglia, it is also of note his three names, and that they represent, because of course I relate to it as well. The names represent not only different periods of his life, but different personalities, or state of being he is. To begin: Ajax. Ajax, easily identifiable, represents the child that lived in innocence up until his fourteenth year. It carries the hope of a fair and good life, where nothing went wrong and happiness was of abundance. It is his inner child, the spark that once lit his eyes.
Tartaglia comes next. Tartaglia comes from the cruelty of the Abyss, of surviving and of the power he holds. It is a title that shows he has survived–the youngest Harbinger, the Tsaritsa's Vanguard: Tartaglia is a shield. Tartaglia is the hypervigilance of his surroundings, the need for bloodshed, to maim any potential threat before it can hurt him. Tartaglia is isolated, above any common soldier or general, disconnected to his fellow Harbingers, unable to see the reasons behind why they act. Tartaglia is a weapon, meant to be used, to serve a purpose in a way he doesn't decide: so unable to do anything other than survive, knowing how to do nothing other than fight, it makes sense to dedicate himself fully as the Tsarita's weapon. Ultimately, Tartaglia boasts his trauma and shows you just how well he can fight, that he is still surviving.
Childe, then, is the remnant of hope. Childe is the mask worn–it is what he introduces himself as, the showing of a cheerful, childlike man. Friendly and confident, willing to protect and keep any promise held. A fierce dedication to his family–parcels of letters and medication sent back to the family that sent their teenager to the military. Childe, willing to sacrifice his health, even like, for the sake of his little brother who unknowingly wandered into mortal. Childe is the want of normalcy. The want to create a place safe enough to do something more than survive–to let Ajax breathe for the first time in years. Willing to sacrifice himself for the sake of those the holds dear, Childe would do anything for the approval that he lacks from his peers and family.
Three separate states of being, three separate all conglomerating into one fucked up guy. A silly guy who puts on a front that to either hide away from his trauma or to boast how he has survived. A constant switch, unable to lice without the other: both Childe and Tartaglia, who hold the dying corpse of Ajax in their arms: one trying desperately to heal him, the other desperately trying to protect him. Ajax needs both to survive, but is given only one or the other, never both, never given any gasp of air to breathe.
The change of names could also hold significance in trying to avoid–another common symptom of C-PSTD. By changing such an integral part of himself, it creates a distance between who he is now versus who he was then. Perhaps it is projection: I, too, changed my name to separate myself from who I was, from the trauma that clung to my life in the past, which makes it of perfect sense to why it is fitting, then, for Childe to boast three separate names. Ajax and the trauma of the Abyss, to Tartaglia and his time spent in the military and ostracized from his peers, and Childe who, hopefully, will rise apart from the names that haunt his every step.
Ultimately, he is just a silly little guy with a shitton of trauma. A jokester whose need to survive slips in every now and then, a constant reminder of the harshness he faced on a prolonged basis. And because of that, your honors he is sooooo me. Me fr. Omg. Me if I didn't get therapy.
also there's a lot of jesus allegations, analogies, & theories going around by childe fans/mains, and who am i if not a lover of catholic imagery??