I've been falling headfirst into Barret/Sephiroth as a ship. You all just have to deal with it for now.
-- Barret not trusting Sephiroth joining Avalanche at all. That he will betray them at some point. Then when looking where Marlene went off to play while he worked finding her sitting with her stuffies and a very worried Sephiroth around her ramshackle play tea set. That Marlene very simply wrangled him into playing with her and he took it as seriously as any mission, like he could get a good grade in playing pretend. When asked Sephiroth stated Mr Teddy wasn't feeling good so he had to step in. He hoped he did okay and Marlene had fun. She did.
-- Barret trying to find Marlene in a game of hide-and-seek. He sees Sephiroth and decides to check since Sephiroth did play with her during the tea party incident.
"I have not seen Marlene," Sephiroth says very carefully. There is a very quiet giggle. Barret peeks and sees Marlene hiding under Sephiroth's coat as Sephiroth very deliberately does not look at her.
-- Marlene holding out a stuffed animal before Sephiroth has to go out for a few days.
"You should keep him," Sephiroth said, "Your dad said Mr Teddy is important to you."
"This is Mr Bunny," Marlene informs him, "He fights off nightmares. He'll keep you safe!" Sephiroth damn near cries at that.
-- After someone mocks Barret for being too soft when it comes to his daughter, Sephiroth immediately rising to Barret's defense. Stating they all need something to fight for other than hate. That Sephiroth hates ShinRa more than anyone, but to fight for a good cause, for someone you love is a more honorable cause than anything.
"I've lost good friends, I've lost everyone," Sephiroth says, "but I see a glimmer of the future I once hoped for with them when I see Marlene. If you don't have that, you're not fighting for any future."
-- Marlene walking around with a beautiful new hairstyle. Sephiroth walks around with the messiest bun anyone has laid eyes on. He refuses to take it out because Marlene worked so hard on it.
-- Barret talking Sephiroth down from a panic attack after Marlene falls asleep on him and he doesn't know what to do.
"I didn't mean for...she just closed her eyes..."
"It's okay," Barret tells him, "she trusts you, she knows you won't hurt her."
"But you're her dad, shouldn't you-"
-- Barret realizing that Sephiroth always covers their retreats. Always. He makes sure as many Avalanche people come back as he can.
-- Sephiroth covering a retreat and suddenly receiving cover fire from Barret. He's not alone.
-- Sephiroth and Marlene have a secret handshake.
-- Barret adopting a shoot first, ask questions never policy when he first meets Hojo. Sephiroth is smitten by then.
-- Barret finally getting a hand prosthetic for his arm when Sephiroth asks. So Barret can fully hold Marlene...and Sephiroth.