There really is no theme, if you want it to have one im good with anything but sin.
Characters? Just use some oc lmao. You can even use a character from a video game or movie. Ive done that.
Sammy: *quickly writes something, runs and drops it by you, and goes back to the swing*
It reads: 'that is quite neat. Is it a learned skill, or did Lucy inject something into you?'
Sammy: *she started getting bothered by some neighborhood kid*
Kid: hah! Freak! You cant even speak, because you have no jaw. *he picked her uo by the shirt* you are a freak.