It's a gorget. Albeit a "discount" version that is mostly ornamental, that often appears on skimpy female armor.
Fate's 'Eri-chan' also wears one on her bikini armor outfit.
Though, these kinds of stripped-down gorgets have a tendency to be tagged choker or collar because of the resemblance.
looking at other examples of gorgets, they seem to be made of metal, while this seems to be a leather necklace with a pendant attached.
In that case, we should eliminate her!Tee hee...Yeah, eliminate!*Unwrap*Bring it on*Trot*
*Trot**Pomf!*Hey, waitー!
What is that girl doingー!?Tee heeNo fair, getting a march on usー!S-Sitting on his lap...!
U-Unforgivable...!Unforgivable
Unforgivable
Unforgivable...Aーh!
I've seen her before!
She's Onii-chan's stalker!Brazenly declaring themselves family and using that to cling all over him...
This is worthy of death...!Hm?Who are you?Stalker!?