Scarlet David/Affiliation

SSSN
Scarlet is a member of Team SSSN.

Sun Wukong
When Sun reveals Blake Belladonna's Faunus identity to Neptune Vasilias, he makes his teammate promise not to tell Scarlet at the first chance he gets, implying a lack of trust towards Scarlet regarding secrets.

Neptune Vasilias
As Sun asks Neptune not to tell Scarlet his secrets the first chance he gets, it's implied that Neptune and Scarlet have a close relationship.