I've heard a lot of things about Joey's sister, and I've finally seen enough of her in action to form an opinion of her. . . . I think she's around thirteen, but I can't confirm that. She seems younger than her age, I know, but I'm assuming that's because their mother really sheltered her. It makes a lot of sense. She does seem like perhaps the slightly old-fashioned mother who doesn't want her daughter to fall in with the wrong crowd. Serenity acts a lot like girls her age did in books written at the beginning of the last century. Specifically in question, "Pollyanna." And don't laugh XD Seriously, Pollyanna is twelve in the first two books, and she behaves very VERY similarly to Serenity. It's rather amusing actually ^^ But anyway, yes, the mother must have raised Serenity in quite an old-fashioned way to try to keep her safe.
I know some people think Serenity is a big twit, but I don't see her that way. I see her as someone who hasn't had much experience in the outside world through no fault of her own and who idolizes her big brother Joey ^^ In my stories, I've tried to portray her as the very kind, sweet, and strong person I see her as. During her adventures with Yugi and the others, she learns many things, both good and bad, and these further shape her personality.
In a short tale I'm still writing, she sees Marik thrown from his motorcycle and shot down when he's trying to warn the others of danger. She is attracted to the Egyptian boy for his bravery and kindness and even kisses him on the cheek in gratitude when she realizes that among the lives he saved was Joey's. Tristan and Duke still compete for her affection and Serenity loves them both dearly, and now she has a crush on Marik as well XD He isn't after her heart, but Tristan and Duke probably will see him as competition anyway because of Serenity's feelings. Though she's much too shy to ever say how she feels openly, Marik has been added to her list of those she goes to for comfort when Joey isn't around. That says something ^^
Anyway, yes, I do like Serenity very much and I have a lot of fun writing for her. I think she's an awesome character ^^ She just needs a bit of developing and I'm looking forward to seeing how the show does that ^__^ Meanwhile, I'll keep doing it myself.