Brenna O'Brien was a pre-War American citizen who was accepted into Vault 114.
Living before the Great War, the infant Brenna, her father Kevin, mother Megan, brothers Brandon, Ryan, Logan, sisters Anna, Kayla, and Molly were all designated to live in Vault 114. Their family, the O'Briens, were assigned to the Residency B3 section of the vault. However with Vault 114's unending construction they may not have been there when the bombs hit Boston.
Brenna O'Brien is mentioned only in Fallout 4.