I guess I don't have much of a fair answer, having only played PSI and PSIV, but my vote goes for Rika.
From the start she's cheery, even after having lost Seed, something that would normally crripple someone, the loss of what was essentially her father.. .. I assume Seed was male..
Anyway, yes, she accepts he is gone and carries on, I don't recall her ever being angry, or showing any signs of hate, something I rather envy.
She is comforting, helping Chaz get through Alys' death, not even mentioning Seed in a kind of "I got over it so can you" way.
She really learns, everything from first leaving the Birth Valley is a new experience, well .. okay not
everything.
All this from a one year old.
Seed really improved the Nei experiment quite well.
Lastly, her combat skills are excellent. Doubleslash is ever useful. Her healing capabilities are second only to Raja, and she has excellent agility.