Actually, I meant the ranger. Horizon Walker is the ranger subclass.
A ranger champion? I don't buy it much. If she had only said 'I fought for Zariel cause she forced me to', why not. But she clearly says she was her champion. And in my mind, a champion is either a paladin or a fighter.
The Horizon Walker subclass includes abilities that aid the ranger in traveling between planes.
This, combined with the fiends chosen as a Favored Enemy, would make her a talented killer of the tanar'ri, killing high-priority targets and just an effective fighter.
Let's say, why not? One more thing I would add in favor of paladin over ranger is the fame of the class. Ranger is more a niche class compared to paladin I think. If you made a poll about what class players would rather play between the two, I think paladin would come first.
But you might be right, it might be ranger. Time will tell.
As well, if she had the ability to travel between planes, why did she need to board the nautiloid to come back to faerun?