Leonardo has some changed to known canon. He was known as "Venus," as a child, but has since came out as Leo prior to the events of the story. While not part of the main story, Leo's gender identity in this story is important to me. In addition, both April O'Neil and Cassandra "Casey" Jones are trans girls.

In-Universe Timeline

November 2013: Leonardo went missing on a rainy night.

February 2014: He was found by the Foot Clan in an alleyway.

June 2018: Day of Leo's first mission, and when he met the Mad Dogs.

October 2018: This is when Story 1 - Enemies takes place.