Disclaimer: I do not own Harry Potter.
He didn't know why she insisted on carrying the tattered teddy bear everywhere, he'd brought her dozens of new toys over the years and she refuses to touch any of them. Now Bellatrix was eight her father Cygnus, thought she was too old to carry it everywhere she went.
But every time he tried to take it away she'd shriek, not a normal child's shriek. No, it sounded like a wild animal being tortured and if shrieking didn't get him to hand it over she'd resort to violence.
Not on him or Druella, but on her sisters.
She'd bite, punch and kick while he tried to drag her off Andromeda or Narcissa.
His wife said it was a phase and she'd grow out of it eventually, but Cygnus doubts this.
Because he knows once Bellatrix's obsession with a teddy bear is over, it'll be because she has become obsessed with something else.
And the pattern will repeat itself until what ever her last obsession becomes will kill her.