Firstly, Welcome DfKimera!

Secondly, I didnt think Rinoa was annoying. I mean, she was just trying to get to be like the others in SeeD even though she didnt really get it. She, Squall and Zell are my most reliable characters. I thought the pairing was cute too. I think she makes up for the non-socialbleness (not a word I know) that is in Squall.
And yea she is pretty key to the plot