soraka's rivals

The fact their ult's are practically complete opposites of one another is probably one thing, though you could also make the case that: "Karthus thinks everyone would be better off (un)dead, while Soraka wants everyone to be alive, healthy, and safe." So conflicting goals, in a sense.

I understand how become rival with But I still don't understand be her's rival.

By the way, I think can befriend with

probably related to a gameplay thing, lol.

soraka global ult vs karthus global ult

they vie for global domination!

Their ults are complete opposites as well as their general themes.

Karthus worships death,while Soraka is a bringer of life.