(Trigger Warning: This mentions habits that lean towards eating disorders.)
Years of acid had wore away at his esophagus. His fingers bore the telltale cold of starvation. It hadn't started off as one, and it never quite accumulated into a diet. It was too far, too deep to be called something as simple as a diet. The first time Kakashi had vomited was at the prospect of his deceased father. The second time, self induced. Plagued by the nightmares of Rin's passing, Kakashi couldn't stomach his own actions much less anything tangible. After a week or so of locking himself in solitary confinement, the teen had been forced out of his idle state by Minato and dragged out.
Kushina had all but shoved food into the boy, horrified at the fact that the boy hadn't swallowed a single morsel of food for more than a week. It wasn't that Kakashi had wanted to starve himself, at least not then. He simply didn't have the motivation or the energy to nourish his body. When Kakashi returned home and Minato satisfied with the boy's wellbeing at the moment, he was left in solitary once more. The food that had felt warm and filling at his teacher house now felt heavy and gluttonous. He began to feel a hot, burning sensation from his core, and he needed to get rid of it. The boy grew restless, he needed to empty himself. Stumbling to the bathroom, Kakashi crashed at the foot of the porcelain bowl, and with the same hand that tore through the organ of his last teammate, he emptied the contents of his stomach into the toilet. Shuddering with every lurch and gag, the boy finally stopped when nothing remained but the bitter slice of bile. An overwhelming sense of euphoria encroached the Hatake child, freeing his mind of death for once. He felt empty, ridded of his sins, and for the first time, pure. Kakashi eyed the whorls of mud in the toilet, wondering if it was messier than the state of his mentality. Food was no longer a priority, it was only then consumed when the boy required energy.
As time elapsed, food was almost a forgotten necessity to the silver-haired male. Almost everything he consumed, he threw up. Rinse and repeat, it was a never ending cycle that Kakashi could rely on. It would not betray him, would not die on him. And he would leave feeling cleansed. When Minato discovered the true extent of damage that had been done to Kakashi's mind, and his less than healthy habits or lifestyle rather, he took immediate action to Kakashi's recovery. His care was short-lived at most, with the appearance of Kurama and the death of the Fourth Hokage. The little progress that had been made shattered and Kakashi fell back into a relapse exponentially worse that the first. By then, he was dead set on his opinion, he did not deserve food.
Kakashi should've died, back when Obito's limbs and soul were crushed, back when Rin's heart was torn apart by ravaging chakra dealt from his own hands, back when Minato died to protect his son and the village. Hatake Kakashi shouldn't have been alive, it didn't add up. Almost a decade of starvation and self-induced vomiting should've murdered the boy ages ago. He should've been frail and bed-ridden rather than consistently carrying out S-Ranked missions. Living on military ration pills was not living. Everyone who cared about Kakashi had long sinced passed on. He awaited his turn, he welcomed death as if he were an old friend. Gai called it slow suicide when he found out, and he wasn't wrong.
Kakashi needed an exorbitant amount of help, he was aware. Truth be told the Sandaime was quite glad when he heard of Kakashi's resignation from ANBU, the boy's reclusive habits were becoming worrisome. It wasn't until one fateful morning, when the Third had head over to Kakashi's flat to assign him to a genin squad that he figured the extent of Kakashi's troubles. The emaciated teen was unconscious, surrounded by a pool of blood speckled bile.
It took three good years of the Third's constant check ins and Gai's constant support for Kakashi to learn to eat again, almost as a toddler would. Three long years for Kakashi to grasp control of his life again. Even then he had his thoughts of relapse. Deciding the man had been alone for far too long, Hiruzen assigned the now adult to a genin team of his own, Team Seven. It turned out, that Kakashi had grown quite fond of the children rather quickly, so when Sakura began to exhibit interests in thinness, Kakashi quickly turned her away from its allure. For once, there appeared to be order his life, meaning to live. He finally discarded his thoughts of undeserving.
His world broke apart once more when Sasuke severed his ties with Konoha, and it shattered when Naruto left to train with Jiraya and Sakura with Tsunade. His life fell into a deep chasm once more, and his old habits began creeping back. All those years of just learning to accept his past had been for naught, he failed Sasuke as a teacher, he wasn't enough for Naruto and Sakura, and this time, there was no Hiruzen to put him back on track. Kakashi slowly began to waste away once more, a feeling of déjà vu clouding his malnourished mind.
This time, it was Sakura who found her former mentor as a shell of the man she looked up to. His silver hair was no longer lustrous, his knuckles bore red scars, his hands themselves as cold as ice, and the man looked as if he had no will left to live. Tsunade was hell bent on saving the man, and another two years passed, his recovery less efficient than the last, but he had learned to cope nonetheless. But he truly was a shell of the shinobi he had once been. He no longer had the cold eyes that cut through steel, nor the warmth that lurked in his eye smiles. He became nothing. The acid that had eaten away at his esophagus had eaten away at his soul as well. When Naruto returned, he almost couldn't recognized the famed man. It wasn't the eating disorder that ended Kakashi, but he was so encroached in his disorder it might as well have.
(A.N)
This was just a small one shot on what could've been a possibility I guess. You don't see too many stories about Kakashi dealing with things like eating disorders so I wanted to experiment a little with his mentality. It's rather shabby to say the least but I hope you enjoyed the piece!