So I recently wrote this whole backstory on why April is teaching the turtles' daughters in my little offspring fanfiction thing called KBYG (I have a whole chapter dedicated to this thingy I made when I was 10)
Here's the copy pasted text from the original Google doc (with the alternate universes in bold) :
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Original (Based on TMNT 2012)
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5 years after their last fight, the turtles retired. But April, being a trained Kunoichi, still fought in New York. After the turtles got affected by a machine who permanently transformed them into humans, they decided to marry the people they loved.
- Karai (who isn't related in the KBYG universe) married Leo at will;
- Raph met Cornélie in a restaurant and the two fell in love with each other, and eventually married 6 months later;
- Mikey met Irma (who he was confused about, since he thought Irma was a robot, but quickly got told that the Irma he already knew was a doppelganger and the one he met was the real deal) and married her around the same time as Raph did;
- And finally, April decided to marry Donnie, who was extremely excited to hear.
After their children were born, April decided to teach them ninjutsu and have them become the spiritual successors to the turtles. She trained them for 15 years and still trains them to improve their skills.
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Wacky (Based on TMNT 1987)
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After the turtles retired, they came across a machine that turned them into humans permanently.
One day, April was fired from Channel 6 and didn't know what to do.
With the help from Splinter, she learned the art of ninjutsu and became a skilled ninja.
Months later, the turtles married some of the allies they knew (Lotus Blossom, April, Cornélie and Irma) and had their children (Miami, Taylorine, Amanda and Nicolette).
After April met the kids, she realized that they could be the next New York ninjas, so she trained them in ninjutsu for years.
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Cool (Based on TMNT 2003)
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The marriage between April and Casey didn't last long and they eventually got divorced. Don decided to create a machine that made him and his brothers turn into humans in order to have more time with the people he knew and loved, including April.
Years later, Leo married Karai, Don married April, Raph married Cornélie, and Mike married Angel.
After their children were born, April decided to teach them ninjutsu after years of training she had herself.
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Modern (Based on Rise of the TMNT)
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After the turtles retired from fighting, April told Splinter to teach her ninjutsu, but when she realized he wasn't a great teacher, she decided she would learn it herself. After years of self-taught ninjutsu training, she became a fully fledged Kunoichi.
The turtles married their allies: Karai (who found a new body after being merged with April for years), April, Cornélie and Sunita (who was finally accepted into humanity even in her yokai form).
After getting married, April made it her mission to train their children to become the turtles' spiritual successors.
She had a hard time being busy every day: she had to keep training ninjutsu while also practicing spellcasting, raising a child (Taylor), hanging out with her best friend Sunita (who was also raising a child, Nicole), and having some downtime with Donnie.
She also had a job: sometime after Taylor was born, April opened an antique shop named "2nd Time Around" and had to manage the store. This made her already busy day even busier.
While she was chatting with her fellow mother friends, her daughter Taylor met their children (Mia, Leo and Karai's daughter; Amy, Raph and Cornélie's daughter; and Nicole, Mikey and Sunita's daughter) and they became friends (and also referred to each other as sisters).
April, seeing this, decided to train them the ninjutsu she was training herself in, to help the girls be skilled enough to have their first fight years later.
