Well here the scenes I'm guess it to be. They all contain Lina and Serene arguing. Yep kinda weird spoilering text but still... It could be spoilers.... or not... (http://www.gamefaqs.com/portable/gbadvance/file/921257/46447)
Wiese Forest Screen 05 at Chest
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Lina : Did it just sink more?
Fia : Maybe we should forget about it.
Serene : Lina's light. She could do it.
Lina : No way! You have wings...
Why don't you try?
Fia : Please, you two, stop it.
Tree Hollow Screen 1 at Tree
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Lina : I want it!
Fia : But, it's too high up to reach.
Lina : Don't worry! Lina'll shoot it down.
Serene : What if you miss?
Why don't I just fly up there and grab it?
Fia : Lina, are you and Serene at it again?
Cierra : Really, you two...
Lina : Ein, can you decide?
Ein : Who should go?
Royal Tomb Screen 4
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Ein : Looks like we can get in from here.
Cierra : I think it's the rear entrance.
Serene : There are steps ahead, so kids, be careful.
Lina : Hey! Who're you talking to, Serene?
"If it is possible to manipulate this thread, then it will be possible to control everything in the world." -Luminous Arc