The Branch in verse 15 is the Messiah. The “she” in verse 16 refers to the city of Jerusalem. Salvation and safety are in store for Judah and Jerusalem because of the presence of justice and righteousness personified in the Messiah. The name given the Messiah in 23:6 is here given to Jerusalem.
Most scholars agree that the name The LORD Our Righteousness refers to Jerusalem. She can have the same name as the Messiah because she reflects that righteousness the Messiah bestows on her. Jerusalem will then be the embodiment of the nation's ideal in the Messiah. The city will be marked by righteousness (cf. 2 Cor 5:21). This [is] what God planned for the city after the exile in Babylon.
-- Gerhard Pfandl, correspondence