You still haven't answered the question.
Why should she be a paladin or hellrider?
Because every single companion you meet so far is of a different class. So as Lae'zel is already a fighter, there is no point in having another one. And as Karlach is on the fighting side but has a story with Zariel, and the fact she comes from Elturel, makes her the perfect candidate to be the paladin among the companions you take on your journey.
