I know. I didn't like it either. It felt forced.
Sometimes I do like new additions to casts, Axl from Mega Man X worked since he filled an archetype that neither Mega Man X or Zero did in terms of personality. MMX is the Stoic Leader/pacifist that fights when needed. Zero was the Tough Guy. Axl was the **** talker. Gameplay wise, X7 was terrible in all aspects, but in X8 he served as the MMX version of Bass.
As for comic books, I liked Tim Drake and Kaine Parker(Scarlet Spider 2) for Batman and Spider-Man respectively for the same reasons. They had different personalities to make them worthwhile.
In Dragon Ball, I don't particularly feel any of the Saiyans after Future Trunks(and by extension, Future Gohan, for the little we saw him.)really stood out either. Kid Trunks, Pan in GT, and Caulifa are bratty. Kale is literally a girl Broly, for better or worse. And Cabba is okay. Goten's lack of screen time is criminal, and this is coming from a guy who's favorites are Gohan and Vegeta.
I like Hit a lot though, since, while he's an assassin, he's shown to have some honor, and likes the fact Goku brought out his joy of fighting.
Anyway, the best way to handle a new Powerpuff girl would if you pulled off the 4 humors theory. 4 Ninja Turtles, 4 pilots for Star Fox, The Fantastic Four.