Who cares??? Rikku is just fun, she's positive, maybe she was more serious in 10... but it was because she thought (SPOILER)yuna was going to die! (spoiler just incase) lol i'd be serious too... In 10-2 all their doing is looking for spheres and having fun... I couldn't stand paine... she was TOO serious...
I love Rikku... She's great, she can have fun and knows how to be serious... So... leave her alone! :P