Well, their personalities could be one factor. If they have contrasting personalities, it can create interesting dynamics in a fanfiction story. For example, if Nergal Jr is more reserved and Mandy is bold, their interactions can lead to both comical and dramatic situations.
I'm not sure of specific Nergal Jr x Mandy fanfiction. But you could start by looking on popular fanfiction websites like Archive of Our Own. Just search for 'Nergal Jr x Mandy' there, and you might find some interesting stories written by fans.
One famous cartoon turtle family feud could be in the Teenage Mutant Ninja Turtles series, where the turtles sometimes have differences and conflicts among themselves.
Hard to say for sure. It might be part of a creative concept for a new comic or animation. Maybe the white lightning indicates some kind of energy or special ability the character possesses. Or it could just be a visually striking design without a deep meaning.