Agent O

In 2012, O is the head of the MIB and receives J, who asks for information on Boris the Animal and his 1969 arrest. She denies access and only says that K also deployed the ArcNet, the shield that stopped the Boglodite invasion, back then. When the past changes and J still remembers K, she is the one who reads the signs: the space-time continuum has been fractured.

In 2019, Molly Wright breaks into the MIB headquarters in New York. Surprised, O sees she escaped neuralization as a child and spent her life looking for the agency. Instead of wiping her memory, she gives Molly probationary status as Agent M and sends her to London. Later, in Paris, after High T's betrayal comes out, she promotes M to full agent and puts H in charge of the London branch, on probation.

Emma Thompson plays O in Men in Black 3 and International. Per Wikipedia, she succeeds Chief Zed, and Alice Eve plays the 1969 O in the third film. In the first two films, the MIB is run by Zed, played by Rip Torn.