Since others are giving reasons, I decided to repost my ranking with descriptions. Color coding, because I love that idea from whoever did that.
Leonie: I mostly just love her aesthetics, personally. I love her short hair and orange hair color, as well as just... the general attitude I’m getting off her. I love her student outfit -I don’t care much about almost any other student’s- , with the gloves, rolled up sleeves, and jacket wrapped around waist. Also a fan of her wielding both a bow and lance. Absolutely my favorite student in Golden Deer by a long shot, and probably will be for the foreseeable future.
Petra: As we don’t have much on her yet, I just really like her design. Her tattoo appeals to me, and so does her hairstyle.
Dedue: He looks cool, and I love how he’s beefy without it being over the top. His earring looks cool, and I love his loyal personality. His skill is great for those moments when you bait an opponent into attacking you so his health is low enough to kill him on your turn. A move I frequently use.
Felix: I like Felix’s hairstyle and his pre-established connection to Dimitri. I also like skilled sword-fighter type characters on the whole, so that works for me. I like the idea of actually benefitting from not pairing him up, or giving him reinforcements as well, since I’m not much of a fan of the whole troops thing.
Ashe: I love Ashe! I like his hairstyle, and he’s adorable. His backstory is endearing, and I look forward to using his skill as often as possible.
Sylvain: I’m a fan of fiery characters, and he looks like he’ll fit one perfectly with that haircut. I also have a weakness for red and orange hair colors.
Claude: I like Claude’s casual outside masking an analytical inside. He’s the most attractive of the three lords to me as well.
Dorothea: She’s cute, and I like her hat. It is a bit out of place, but it somehow doesn’t break the immersion for me. I like her looking up to Manuela as well, since it creates an in-universe connection already. I also like both her singing background and skill. I think both will be incredibly useful.
Ferdinand: I’m generally a sucker for red and orange-haired characters, so there is bias here. I also like his whole rivalry deal as well, and his role as chancellor. Skill wasn’t incredibly useful when I used Subaki, but we’ll see.
Bernadette: I think she’s adorable, and I like her bookish reserved nature. Also, Paranoid sounds like a great skill
Dimitri: I love Dimirti’s chivalrous personality, and the whole shadow bit. I also love non-heavily armored lance users, so he’s just great. The only thing keeping him from being A-Tier -or even S- on this list is the hair.
Annette: I find her fairly adorable, and I find her background endearing. Otherwise pretty neutral.
Lysithea: Her whole prodigy thing is pretty interesting, but it’s about all that bumps her above C for me. Her skill should make her fairly versatile, which is also going to be really nice.
Edelgard: She seems cool, and I like her outfit, but I’m not much a fan of calculating stone-faced characters. Until I get a more distinct sense of her personality, this is where she’s staying.
Linhardt: Mostly here because he seems neutral to me. I don’t particularly like or dislike him.
Mercedes: She’s pretty, and her backstory of being a commoner who was once a noble. That said her whole kind and motherly seeming demeanor is fairly meh to me. Not my style. I do like her skill, though.
Ingrid: She’s reasonably attractive, but still neutral to me.
Hilda: I love her hair and the fact that she reminds me of Serra, but I don’t like her personality as much. It’s probably just that I don’t like the personality in general and Serra is just nostalgia from when I did, but eh. Useful skill, even if the basis for it isn’t particularly to my liking.
Hubert: He looks creepy. I don’t like his hairstyle, I don’t like his eyes, and I don’t like people with a “victory at any cost” mentality. Neutral on his skill. I don’t know how important gambits will be, nor how often I’ll use them.
Caspar: I hate his hairstyle, and I didn’t like the over-the-top hero personality in either Arthur or his son. Unless I find he’s more subdued than either of those, he’s where he sits.
Lorenz: I’m not a fan of snobby characters. His whole look screams it to me, and I hate his hairstyle. I’m not much a fan of the whole troop system, so his skill is pretty negative to me as well. Also don’t noble characters who repeatedly try and fail to court women amusing either.
Raphael: I don’t like his rounded face, and I don’t like how his shirt looks like it’s ready to pop off. He feels too big for me, and his double-minded nature is one I don’t like either. Not an awful skill, but unless his luck is pretty high seems inconsistent. Also... The implications of it being him just finding food randomly on the battlefield is... Ech.
Ignace: I just... personally really don’t like his(?) whole look. Especially the hair. May change once there’s more info.
Marianne: Given out only shot of her is inside a diamond shaped picture, I don’t really even have a good image to work off of.