Geez Mythic, don't hold back whatever you do xP I'll split this up as best I can.
Q1+2:
Gwylan's pretty pleased with his team, he feels they've got a well rounded set up in pretty much every area: combat range, styles, personality, etc, and isn't overtly concerned about their being any problems.
He does his best to like them all equally, but he's fonder of Aurora than the others, if only because she's more willing to match his playful and flirty attitude. He likes someone that's willing to go toe to toe with him in anything, so she does appeal to him a bit more than the others. She confident without being a giant bitch, and he appreciates that. And she's easy on the eyes, which certainly isn't gonna damage his opinion of her, being a teenage boy.
He likes Tina, cause she's nice and a nerd, who he finds amusing and fun to talk to. The fact that she plays video games is a bit of a plus for him, cause he does have an interest in them, though not to the same degree, so that'd be a good bonding point for them, he hopes. Her lack of confidence and social awkwardness is a bit concerning for him, because he's not entirely sure what may upset her, but he hopes to bring her out of her shell a bit.
Nero is probably his least favourite, but that doesn't mean he doesn't like her, just that she's a bit harder for him to interact with than the other's. She's quiet and honest, but he doesn't feel like he has all that much in common with her, especially as his normal methods of getting to know someone don't seem, to him anyway, to appeal to her. He'd like to get to know her better, and hopes they can find something to bond over. And her snake's pretty cool too.
Q3
It really depends on what potential is meant. In combat, he'd probably say Aurora, due to her semblance, and how many potential uses it has, whereas Tina's and Nero's are straight forward or too science-y for him, respectively.
As a person, it'd probably be Tina. Aurora has, in his eyes anyway, reached a level of growth as a person that anyone would be proud of, and Nero, while not as confident as Aurora, is willing to stand by her opinions and ideals, which is no small feat. Tina, though, is limited by her lack of confidence, and he feels that, if she overcomes that, she has more growth potential in her than the rest of the team does.
Q4
He does, and it's very related to the reasons mentioned above. Neither Tina or Nero, in his opinion, have enough confidence to lead the team. They're too reserved for people to feel safe trusting with that level of power. He doesn't think they'd be bad per se, but he doesn't think they're ready for that sort of responsibility. As for himself, sure, he's confident, and in his own mind anyway, charismatic, but he doesn't have the mind for it. Planning and tactics are not something he feels strong with, outside of a small play in a fight maybe. He feels he's too straight forward in his thinking for the job.
Aurora, though, is the most obvious choice. She's smart enough to lead the team and confident enough to hold them together. She's also emphatic and kind, which certainly helps, as Gwylan wouldn't trust someone who didn't have some goodness in them. She may be a bit rough around the edges, but that's why they're in training. A few months, maybe a year, and any issues with her leadership and their teamwork while be gone, by virtue of working together and getting to know each other. And Aurora is the kind of person that, he feels, will encourage that sort of growth, which in turn will help them grow as people.