Mental Diver

Manami Sendo, a girl attacked by a psycho diver, is in a deep coma. In order to save Manami, Professor Ogashira of the World Brain Foundation invites Akira Nekuta, a young psycho diver, to enter Manami's mind. In the face of the evil psycho-diver who calls himself Nekomu, Akira and Manami's minds suddenly merge and transform into a being who calls himself Anima Animus.
Having defeated Nekomu, Anima Animus will continue to fight against the evil secret society, Nightmare, as the strongest force in the World Brain Foundation.

(Source: Mangapedia, translated)