Marlene is the adopted daughter for Barrett. He decided to look after the child when he felt responsible for Dyne's death. (Dyne is Marlene's real father) during the North Coral Fire (the same fire that destroyed Barratt's hometown and killed his wife Myrna). Barrett in FF7 actually found out Dyne didn't die but he did have to have a Gun arm (Dyne's Gunarm was on teh opposite arm that Barratt's was) and at thier last encounter Barratt blames Dyne for ruining the town because he never returned, he just abandoned them. Barrett felt betrayed so Barrett Killed Dyne.
Actually, Barret didn't kill Dyne. They fought, yeah, but Dyne said something about how even if he were to go back to Marlene, she wouldn't even recognize him, so he commited suicide by jumping off the cliff...
....or something like that. >_<
Yay! I finally saw Advent Children! ....but it wasn't subtitled so I'm still not 100% sure what the hell was going on, haha.
But from what I saw, I absolutely loved it! ^_^