Diana Prince/Wonder Woman: Diana enters the rest of the world as a Fish out of Water, a lot of energy, and a certain obliviousness to how she comes off. She lights up when she sees a baby in the street, even squealing "Oh a baby!" When trying on clothes, she practices various combat poses. Steve buys her some ice cream from a vendor and she is almost brought to tears over how good it is.
Steve Trevor:
Steve is quite endearing in how smitten he is over Diana, to the point where he breaks into a cold sweat whenever she brings up sex or flashes her thighs.
Despite the disturbing Grand Theft Me, Steve is gosh-darn adorable when exploring the new 1980s world. He does a fashion show for Diana, experimentally tries an escalator on her behalf, and goes Squee! when she says they need to steal a plane to make it to Cairo.
Barry Allen/The Flash:
How else do you describe a person who jumps with joy at the chance to be in a superhero team to make friends? And then asks to keep Bruce's Batarang and explores the Batcave with the biggest awe on his face? His bumbling reaction when left behind with Commissioner Gordon by the rest of the Justice League's Stealth Hi/Bye also qualifies.
Special mention has to go to the "sorry" face he makes when Aquaman points the finger at him after they collided during the fight against a resurrected Superman. The way he awkwardly waves at Alfred is another moment.
Arthur Curry/Aquaman: He's pretty stoic for the most part, but when Mera kisses him, Aquaman briefly loses his swagger. Then when all the ocean-dwellers hail him as the new king of Atlantis, he smiles through gritted teeth with obvious embarrassment.
Mera: It's not to a significant extent, but Mera does show a quirky, child like wonderment when she and Arthur are in Italy and taking in the sights.
Alfred Pennyworth: Alfred is extremely giddy every time the energy-dissipating gauntlets that he built for Batman work. Throughout the Snyder Cut, he’s in a much more chipper mood than he was in Batman v Superman, at one point happily humming to himself while he pours himself a glass of whiskey, or teaching Wonder Woman herself how to properly prepare tea.
Helena Bertinelli/The Huntress: She tries so hard to seem intimidating and cool, but between the sight of her practicing her introduction and her loud displeasure at being known as "the crossbow killer" instead of her much cooler-sounding name, she comes across as too cute to be scary.
The Suicide Squad:
King Shark is practically a little kid in the body of a giant shark man. His simple, innocent attitude has made him an enormous fan favorite.
Polka Dot Man already comes across as awkwardly sweet in general, but his downright groovy dance moves define this trope.
Peacemaker's horrifically awful but well meaning dance moves help him apply as well.
TDK's enormously pleased and genuinely determined expression as he offers his team minimal, awkward help is strangely charming.
The otherwise manly badass Bloodsport has a horrible fear of rats that's both humorous and cute.
As always, Harley Quinn is beloved for shamelessly bursting at the seams with energy. The often happy and sleepy Ratcatcher is so forthrightly sweet you can't help but like her.
