You've watched his body become weaker and weaker for months now, breaking down and decomposing even as it breathes on. You've heard him cough and choke and seen him unable to stand without support. You've seen the spirit leak out of his eyes and you've hated every moment of it. But now it's affecting his soul and you can see how weak he is and you're going crazy with the realization that as his body gives out, his soul is also giving out. You don't think you can take it anymore.
He used to be the strongest person you've ever known—he still is, really. He's breaking down before your very eyes and yet he still defends his family and his friends with everything left in him. He is still brave and loyal and determined and everything you've always admired about him but never admitted. Maybe he is more brave and loyal and determined now; or maybe you can just see it better now. Either way, you have never seen someone so weak and so strong at the same time; you don't really know what to make of it.
You watch the others give up hope—his family, Rukia, Orihime, Sado…. You even see Ishida bow his head one day as he accepts the fact that it is only a matter of time now. Yet you remain in firm denial, hoping without hope that they are wrong and that he can still come back from this. You watch him closely in battle and jump every time he coughs or pauses to catch his breath, but you never admit to yourself that he is actually dying. You still argue and yell and turn everything into a competition even when he lacks the energy to reciprocate. You still visit the real world randomly and sneak through his window with a jug of sake even though you know he won't be able to stomach it. You still challenge him to sparring matches whenever he is in Seireitei even though on some days you could wipe the floor with him without really trying. You never do, though. You live as if nothing is different, as if nothing has changed. You live as if your best friend isn't dying.
You watch as he gets worse and worse, and eventually even his Shinigami form is unable to fight or train or leave his house. He comes to Seireitei one last time to say goodbye, and you avoid him as he visits all of his friends—Rukia, Hitsugaya-taichou, Matsumoto-fukutaichou, Ikkaku, Yumichika, and all the other Shinigami whose lives have been changed forever by this now feeble human. He finds you after he has seen everyone else; he knows exactly where to look and you aren't surprised. He sits beside you, moving entirely too slowly, and you both sit in silence for what could be years or seconds or centuries. In the end nothing is said, but as he leaves he sets something on the roof tiles beside your leg. You don't dare to look at it until long after he has gone, and when you do you understand for the very first time that you will never see him again.
You skip the funeral, but you go to see his body one time before it's buried. You stand next to the casket for an hour, staring at his pale, gaunt face. You clutch his gift in your hand so tightly that your fingers go numb and try to find the words to say goodbye. There are none, and before you leave you place his Substitute Shinigami badge beneath his crossed hands—he had left it for you, but you're sure that it belongs with him. You take one last look at your friend's face and leave his house for the last time.
You are unable to truly say goodbye for a long time, but you watch the world go on without him and you know that that is what he would have wanted. You go on without him, too, fighting for the things he always fought so hard for. You become stronger as the years go by and when you become captain you visit his grave for the first time and are finally able to truly say goodbye. You thank him for pushing you to work harder and become stronger and for teaching you what it means to truly fight for what you love. When you leave you are finally at peace.