dead friend

I had a friend called Kimaro when i was 8-years-old. He was a nice small boy, really polite and funny, we would usually play board games or other games together.

(we called each other different names like i would call him 'Kim' for short and he would call me 'Muki'. for short. (my real name is Mukley)

we didn't see each other for a few months.

and when i seen him again, i told my mum but then she would always tell me that he was not alive but he was playing with me every day.
how could he be dead?
then how can i could see him?