AU Explanation, before we start.
This is sort of a smash up between the 2012 cartoon and the new ROTTMNT- settings and character designs are like ROTTMNT's but the characters' personalities sort of depend. More on that later. The Kraang do not exist and it never explained where the magic weapons come from.
Donnie, years ago, joined the Foot Clan- the 2012 version- and Shredder due to resentment and such. He acts as second in command next to Karai and also is the brain. He dissects mutants in his free time. He is clever and is basically a psychopathic evil genius. Not really sure if he wants to kill and dissect April or fuck her into the nearest surface.
April O'Neil is a tech genius and her dad is a world famous scientist, who does not pay any attention to her. She has inherited her Indian mother's looks but her Irish heritage shows through the red tint to her hair. She met Splinter and dragged her childhood friend Casey Jones into the battle and ninja training. Suggested to be fully human, but has an odd connection to the turtles' magic weapons.
Casey Jones is...still Casey Jones. He and Raph have loud crushes on each other.
Mikey is still the artist of the group and master of gymnastics. He has the magic fireball of a weapon. Doesn't talk about Donnie much. Gets excited about every new thing about the surface.
Leo and Ralph share the position of leader. Leo makes jokes but gets really serious in battle, especially when against Donnie and has the portal sword. Raph gets angry in battles against Donnie but is mellow outside battle and has the avatar tongas and his backup weapon of the sais.