"Well....not exactly..."
Its more like the writers forget they exist sometimes.
'So, they only exist sometimes?.. Are they some kind of magician?'
'I'll be worried if my baby brother disappear sometimes. Whenever that happens, he's usually causing trouble.'
Astro Boy
Jetter Mars
"There's Cobolt and Chi-Tan...though sometimes they exist, and sometimes they don't."
Astro Boy
"Wait... YOU GOT OTHER SIBLINGS!?"
"How many siblings do you guys have?"
Jetter Mars
"Uran, how do you think our other siblings feel? Most people forget they even exist."