After the titan's origins have been discovered, Commander Katsuki, and his team were sent out to find out more about the titans and how people become them. Katsuki's group gets the feeling they were being watched, then Katsuki spots a person hiding in the tall grass. Katsuki demands they show themselves, as they were outside the walls, which was dangerous and as the scouting unit they are meant to protect others.
The person introduces themselves as Izuku Midoriya, a boy who lives OUTSIDE the walls. Confused, Katsuki and his friends question the boy. After a few questions and off-handed insults at the walled city, Izuku leaves Katsuki follows after him, with the blonde's friends close behind. Katsuki loses track of the boy since Izuku was too fast. After looking around for a while, Katsuki sees Izuku high up in the tall red oak trees. Higher than the titans could reach, and higher than the scouting unit's gear could reach. Izuku wasn't alone, a blonde girl called Himiko Toga was with him. They jumped from the high branch, Katsuki was shocked to see they landed with no injuries, despite how high the jump was. Katsuki was led by Izuku and Toga to an entire village of people living outside the walls. This village was underground and only accessible through secret passages hidden in the high branches of the surrounding trees. There he meets the son and daughter of the village leader, Shouta Aizawa. The son was named Hitoshi and the daughter was named Eri. Eri left to play, and Hitoshi gave Katsuki a history lesson, but not one he expected...
I have a head-canon that at the beginning of the age of titans, there were humans who actually wanted to study the titans, hence why they learned all they did about the titans and the weak point on the titans neck. In my au, these researches were separated from the other humans and became so obsessed with their research they found a way to harness the titans' power.