Never did care much for Rosa; she's too much of the typical damsel in distress character for my taste. Onion Knight/Terra is an absurdly crack pairing, but I like Terra a lot so I'm voting for that one.
I'm not totally fond of Rosa either tbh, but I found the pairing of Onion Knight and Terra just downright weird. I know the Onion Knight is not a child, but it feels like he is, so pairing the two of them up...Yuck.