So, you guys from Jeresy?

When Splinter was escaping from Draxum, he accidentally dropped the kids and ultimately managed to save only Mikey. Donnie was left with Draxum, Leo was found and given to Big Mama, and Raph ended up under The Foot's feet.

Mikey

Mikey lived a completely ordinary life with his father in the sewers. He was an energetic and happy child, loved to draw, dance, and skate.

However, over time, he began to get lonely. He didn’t really have anyone to talk to aside from Splinter.

Splinter decided to go to the Hidden City so his son could interact with other mutants. While Splinter worked construction jobs, delivery, in restaurants, and wherever else he could to earn money, Mikey socialized with other workers and entertained himself. As a teenager, he started going shopping on his own. He loved chatting with all the vendors at the market. When he was about 7 years old, he met April, who got lost in the tunnels.

In general, Mikey lived a pretty normal life, not worrying about anything – just having fun, watching movies and training. He hung out with April around New York and Hidden City, doing silly “ninja” things and eating pizza. Mikey did feel lonely sometimes, like he was missing something crucial, and never forgot his imaginary friends from childhood – Red, Purple and Blue.

Raphael

Raph grew up with the Foot Clan. They found him on the street and took him in. He had an incredible talent for fighting and brutality, and they were happy to nurture that talent.

One day after a big argument with his gay uncles, Raph ran away. He loved animals, and when a stray cat came to ask for food and pets, he tried to hug it to calm down. But as a 6-year-old child capable of crushing stones with his hands he couldn’t control his strength and broke the cat’s spine. He was very careful about hurting small friendly things after this.

When he was about 9 years old, a human girl named Cassandra was brought into the clan and handed over to Raph. That's how he got a troublesome younger sister whom he was in charge of.

Raph is incredibly loyal to his "family," but he treats the whole villain thing more as a fun game than anything serious.

Donatello

Don stayed in Draxum's lab and was raised directly by him. Draxum had high hopes for him, very specific ones too. There was an image of the perfect mutant warrior that Donnie was supposed to embody. However, Donnie only excelled in science and lagged behind in martial arts and mysticism.

When Donnie was about 9 years old, he met Leo at some fancy family gathering. After that, Mom and Draxum began organizing social outings for them that Donnie (and Leo) couldn't stand. However, thanks to Leo, Donnie got his hands on his first piece of human technology. Draxum hated phones and computers, but Donnie fell in love with them at first sight.

Thus began Donnie's obsession with robotics and programming, sponsored by his brother. In his teenage years, Donnie went through a rebellious phase and started constantly arguing with Draxum about technology. It all culminated in a huge fight after their 16th birthday with Leo. Donnie showed his father all of his endless inventions by fighting him and almost nuking the lab, and when Draxum rejected them again, he couldn't hold back his tears. In the end, Draxum gave in.

Now Donnie has a whole business, sponsored mostly by Leo, who in turn gets to own a lot of his genius tech. Leo also finds customers for him, since Donnie hates talking to the rich assholes.

Leonardo

Leo was brought to Big Mama, who decided to raise him as a replacement for her beloved Lou Jitsu. The first 5 years of Leo's life were completely normal. He had a nanny who raised him, fed him, and played with him. Leo never saw Big Mama up close or interacted with other people, but he was too young to care.

When Leo turned 5, Mama visited him. She showed him recordings of Lou Jitsu's fights, and Leo was inspired. Subconsciously, he realized that Mama was setting an example for him. From then on, Leo began practicing martial arts every day. Aside from training, nothing else was expected of him, so in his free time he did whatever he wanted. Mostly he watched recordings of Lou Jitsu and explored their large country house.

When Leo was about 7, he started playing chess with his master. Leo quickly understood the rules, and after just a few months of constant play, he won. Then he kept winning again and again. About a week later, Mama came to visit him. She suggested they play a game. Leo, of course, lost, but Big mama still smiled at him. She even patted him on the head! Leo would chase that feeling for the rest of his life.

When Leo was 9, Mama took him to a rich people gathering for the first time. There he met Donnie, and from then on they became brothers. At such fancy gatherings, Leo had to meet and make friends with many wealthy yokai, and especially their children.

When Leo was 12, and everyone knew about his closeness to Big Mama, he was kidnapped. Leo didn’t know if Mama would come to save him, so he escaped on his own. When Mama found him, covered in blood, something changed in her face. It was the first time she picked him up and cradled him in her arms.

When Leo (and Donnie) turned 16, Draxum gifted them mystical weapons. Donnie didn’t like it and had a huge argument with their father over it, but Leo was happy. At first, he struggled to create portals, but after several months of training, he finally succeeded. From then on, Mama began assigning him to deal with more and more important tasks.