Short answers, pulled from the story.
Aoba Tsuzaki is the protagonist of the original Jinki manga. She was a child who spent her days assembling plastic model robots before being taken to a secret military base in Venezuela on the 1st of January 1988.
Kokushou was once known as Hino Hakuya, a boy who loved his life and his partner Tsuzaki Shizuka. His descent into nihilism began when the Venezuelan military held Shizuka hostage to force him to crush a peaceful riot with the brutal force of a Jinki.
Akao is the protagonist of the Jinki: Extend manga series. She was an amnesiac who remembered nothing from before three years ago and was in fact a clone of Genta's late wife, Akana Ogawara, created by Kokushou out of spite for his former mentor.
The conflict in Venezuela took place during 1988. Aoba and Ryouhei were taken to a secret base where they discovered a new mecha designed to fight the Ancient Jinki.
The story shifted to Tokyo, Japan in 1991, marking the beginning of the Jinki: Extend arc. This period, known as the Lost Years, saw the conflict escalate with the involvement of the Hachishoujin and the Angel unit.
The anime adaptation was animated by Feel and aired from January to March 2005. The 13th episode never aired on television and was released as an OVA, highlighting the differences in the narrative.