Oroku Yori
Her backstory is told in Villain Mini-series #5: as a young girl, Karai found the Ashi no Himitsu, a book detailing the secret history of the Foot Clan, in the library of her father, Oroku Yori.

April is kidnapped by the mutated version of Baxter Stockman. Baxter threatens to mutate her, so the Turtles come to rescue her. When the Turtles come, it is apparently too late. Baxter lowers April down from the ropes that bind her and hang her over a giant container of mutagen.

Donnie
Donatello: Out of all the Turtles, Donnie is the one April is closest with. He has a HUGE crush on April, and fell in love with her at first sight.

April O’Neil is a deuteragonist in Teenage Mutant Ninja Turtles. She is portrayed as a best friend of the Turtles and the love interest of Donatello, who first met them when when they saved her from the Kraang, who needed the psychic Kraang-like powers she had gained when she was born for their invasion on Earth.

As the series progresses, Leo and Karai start to have a romantic relationship, which is seen in When Worlds Collide.

But in the Archie comics, she actually became a mutant turtle. In the 1994 winter Archie special, April was mutated into a turtle, making her the first official female mutant turtle introduced to the series, since this was three years before Venus de Milo debuted.

Apparently, the trailer suggests April O’Neil’s father, along with Shredder, helped create the ooze that created the turtles. Read more about the Ninja Turtles movie origin below. The ooze that Shredder made… with April’s dad. Yes, the VO is Shredder telling April he and her dad made the turtles.

Kirby is April’s father and husband of Mrs. O’Neil. Kirby was kidnapped by the Kraang to help them perfect the mutagen, but Donetello broke him out. He was soon mutated into Kirby Bat when the Turtles dropped all the mutagen canisters out of the Kraang Stealth Ship.