Ooh... fears or motivations. Ok. Let's see... I think I'll go with motivations!
Samara's a kind soul who genuinely wants to help people. She uses her insane amount of powers to do that, and ends up going on a wild journey with a dragon as they run from a god!
On the other hand, Penn is motivated by curiosity. Which sometimes gets her into trouble. Especially when she discovers a hidden lab that makes her something more than human!