"Certain is it that there is no kind of affection so purely angelic as of a father to a daughter. In love to our wives there is desire; to our sons, ambition; but to our daughters there is something which there are no words to express."
Little Marlene
Marlene is Barret's adopted daughter. Her real father was the tragic man named Dyne and her mother was Eleanor. Marlene from my guesses was just a new born baby after the incident because I believe she was 4 years old in the start of the game's plot. I'm also not sure if she knows that Barret is not her real father but it really doesn't matter because she is obviously attached to him like any daughter would be. Since she basically has no memory of her real parents, it can be said that this fact prompted Dyne to commit suicide rather than try to take her away from Barret. Barret is always so happy to see her and is very protective of her. When Aeris gave herself over to the Turks in replacement of Marlene's safety, Marlene is later held ransom by ShinRa spy Cait Sith. She is later left with Aerith's adoptive mother.
Barret was sickened by the thought of Marlene being in danger. When they were finally reunited after a tense period of confusion, Barret was relieved to see his precious and beloved daughter safe.
Elmyra: You're her father!? How in the world could you ever leave a
child alone like that!?
Barret: ...please don't start with that. I think about it all the time.
What would happen to Marlene, if I... But you gotta understand
somethin'... I don't got an answer. I wanna be with Marlene... But I
gotta fight. 'Cause if I don't... the planet's gonna die. So I'm gonna
keep fightin'! But, I'm worried 'bout Marlene. I really just wanna be
with her... always. See? I'm goin' in circles, now.
Elmyra: ...I think I understand what you're saying... She's upstairs
asleep, why don't you go and see her.
Barret: I'm so glad...... I'm so glad you're all right...
Marlene: Daddy, don't cry. Your whiskers hurt!
I was really happy to see that Marlene's role in the Advent Children film was relatively large. In it, it is she who recounts the story of Sephiroth and the events of Final Fantasy VII to the viewer. She is also close to the little boy Denzel, caring for him when he is afflicted by Geostigma. She goes with Tifa to Aeris's church to look for Cloud, and is subsequently abducted by Loz and taken, with the other children, to the Ancient City. There, she witnesses Kadaj's manipulation of the Geostigmatic children, including Denzel, and is forced to watch as Cloud and Vincent take on Kadaj, Loz, and Yazoo. By the end, she is safely back in Edge with Denzel. In the last scene she is seen holding Barret's hand, symbolizing that their bond is still strong.