There aren't really any easy champions, just champions that are less mechanically challenging, but rely on good decision making since they can't escape or fight through difficult situations with outplays since they're one dimensional.
Riven is very mechanically challenging. If you don't take animation canceling into account, she's actually not that hard or unpredictable. However, animation cancelling (which ranges from easy to I-Might-As-Well-Press-Random-Buttons combos) catches people off guard and helps you survive.
Actually, most champions that rely on mobility are generally hard to use like Lee Sin, Riven, Vayne, and Lucian. It's easy to use mobility to just catch up or run away, but confusing people, like you should be doing to dodge skillshots, doesn't happen with just button mashing.
Tl;dr Riven takes skill. Katarina doesn't.