Theo Ngoni is a main character in the last two books of the Mortal Engines Quartet.
He is first introduced in Infernal Devices as a fellow slave of Wren Natsworthy on Cloud Nine. Originally he was a Green Storm suicide bomber, but was captured when his "Tumbler" failed to detonate when it was dropped on a suburb. He was saved by the people of the city, but sold into slavery. Theo is ashamed that he did not go down fighting, but later realises that he was not willing to give his life for the Green Storm; he did not really believe in their cause, but had been brainwashed by propaganda.
Through a variety of mistakes Boo Boo Pennyroyal, mayoress of Brighton and wife of the former Professor Pennyroyal, comes to believe that Theo and Wren are romantically involved. This ultimately leads the two to become friends and help each other escape Cloud Nine. Ironically, this is probably when their actual romance did begin. After the destruction of Cloud Nine, their subsequent capture and escape from the Green Storm, and the witnessing of the duel between the Stalker Fang and Shrike, Wren and Theo escape with Tom Natsworthy on the Jenny Haniver.
In A Darkling Plain, Tom and Wren become air traders, while Theo returns to his home in Zagwa. He foils an attack on Lady Naga by Green Storm traitors, and is recruited to escort her back to the safety of Tienjing. He fails at this - their airship is hijacked over the Sahara desert, and Theo is captured by a Traction City and sold into slavery again.
A chance encounter with Hester Natsworthy and Shrike frees him, and he convinces them to help him rescue Lady Naga.
They save her from slave-dealers on Airhaven, and escape over the front line into Green Storm lands, but Theo is separated from the others when the airbase they are attacked.
He trudges across the plains with a number of defeated soldiers, and comes across the ruins of London. Wren had told him in a letter that she was heading there with Tom, and he decides to investigate it. He discovers her alive and well with the surviving Londoners, and they express their love for each other.