Um, this is the reason why you thought Lauren was sad. In Samurai Forever, she was crying. The reason why she was crying is because she was struggling to give the last of her symbol power to Jayden. So, now I get the 2nd clue. You said " You don't see me that MUCH". That one is Lauren. But I don't get it. If the first is Serena, and the second is Lauren. If they are combined, they are Laurena! But that doesn't exist. So, I don't get it.
It is Serena.One reason is that in Sticks and Stones in Emily's flashback it showed Serena crying and she is very sick so she doesn't want to put Emily in danger.She only shows up in 1 or 2 eposide.It isn't Lauren because why would she be sad?she got to see her brother again.It isn't Dayu because she was shown in basicly every epsoide.So the anwser is Serena.