There is a .caster member you can check. Buff[].Caster I think it is. Caster is the name of the character that cast the debuff
I tried doing ${Target.Buff[Boastful Bellow I].Caster.Equal[${Me}]} which will return true if the character is the first one to cast the debuff but will return false for any subsequent characters that cast it. It does not seem to check debuffs past the first one applied.



