Tadashi doesn't know why he became a ghost but he's glad he stayed. That way, he can watch over his younger brother who, by the way, seems eager to throw himself in suicidal situations without any regard for his safety, the knucklehead.
(He's also guilty and sad because Hiro and Aunt Cass are hurting and there's nothing he could do to help them. He wishes this was all just a bad dream and why can't he just wake up, his brother is crying, why isn't he right there comforting him-)
His friends visit the cafe sometimes. They act the way they usually do but he sees the way their smiles and actions are a little strained, a little forced, as if trying to pretend they were okay.
It hurts him to see them this way so he tries to pretend that they actually are but it's painfully obvious that they're not.
Tadashi worries about his brother the most. He knows that Hiro has trouble falling asleep at night and that he usually wakes up screaming in the middle of the night because of nightmares. He knows that Hiro dreams about fire and a desperate "No!"
It doesn't take a genius to figure out whose death he was dreaming about.
Tadashi also knows that Aunt Cass cries every night and it's all for him and he wants to tell her to stop it, please, because he's still here -
She wouldn't hear him anyway.
It was never supposed to be this way.
He wasn't supposed to die that night, fire everywhere, consuming everything. (Even him.)
No one should have been grieving for him, least of all, his precious little brother.
(Not Hiro, please, he thinks. Never Hiro.)
Tadashi knows he can't protect Hiro forever but he still likes to think he can, even though he's dead. (He's not really gone though, despite what they all say. Of course, they can't see him anyway so they wouldn't know.)
He'd seen the way Hiro's innocence slowly faded over the years. He laughed less when their parents' died. He smiled less when the pressure of being a genius prodigy weighted him down.
They never disappeared though. Tadashi made sure of that. He would do anything to preserve Hiro's happiness, even at the cost of his own.
(He always was one to put others before himself.)
But when he died, Hiro stopped smiling altogether. It was so, so unfair and so, so wrong.
Why doesn't anyone ever notice that his smiles are all fake, he wants to scream. Please, Hiro. Stop it. Stop it right now.
Please stop, he pleads to someone who wouldn't hear him as tears slip past his cheeks.
Where was the little boy with the beaming grin, he thinks, why did he disappear?
There were times when Tadashi would forget himself and reach out to his brother. It always ended up badly, an unpleasant reminder that he wasn't alive anymore and he would hate himself more for dying and forgetting.
He knows that the others would take care of Hiro for him from now on, though.
(He still should have been the one taking care of him.)
It is almost enough for Tadashi.
Almost.
His eyesight blurs and he raises a hand in confusion, realizing that he's crying.
(Please, I want to live. There's still so much I want to say, still so much I want to do. I'm not afraid to die but I'm afraid of leaving Hiro behind.)
(Just this once. If not for me, then for Hiro, please.)
Everything fades into nothingness after that.