Oh geez, where to start... I guess from the beginning.
My number one suggestion to you is to pay more attention to head angles. You cannot mix and match head angles.
For the sake of this post, I have doodled on your image: (Blue circles indicates problem areas)
Character A: Hair and face angle do not match. Hair is coming down far too much on the left (our left) than it should, given the supposed head angle. Also, do NOT mix FE6/7 and FE8 colors. It just doesn't work. You're going to have to make the clothes' shading darker to make it fit with the FE8-like border color. Common newbie mistake; I used to do it myself before I realized how awful it makes things look. The green shading on the shirt (and possibly the purple shading) needs to be darker.
Character B: There's kind of an unspoken rule about FE characters in that the older they are, the smaller their eyes and the larger their chins (if male.) Noah's eyes look a bit too young to fit with that face. Also, the face and head angle is awkward. The face looks straight but the head is tilted.
Character C: The hair near the face is kind of weird, and I want to say something is really off with that far eye. I don't think it's in the right spot, but I can't really figure out what it is that's bothering me, either. It's probably another incompatible angle. Actually, on second look, it's definitely an incompatible angle. The close eye is fine, though, I think.
Character D : Pretty good. The darkest shade in the hair needs to be darker; it's too close to the second shade. The clothing needs to have darker shading, because again, FE6/7 colors and FE8 colors don't work. Also, he's missing border between the back of his hair and his neck/clothing. Also missing a pixel of border on the other side of his neck, next to the beard.
Character E: I...am honestly not a pro with spriting, and I don't want to lead you in the wrong direction. There's something up with this one's face, but I can't figure out what. However, I have circled a place in her hair where there's an unknown line going through it that isn't going in the direction of her hair. Remove this line, and it'll look better.
Characters F and G: Pretty good. I see no problem areas with either, although the beard on G looks a little too angled.
Character H: This is what I'm talking about with paying attention to head angles.
Look at them closely. See how Lute's head is straight, if not sliiiiightly angled back, and she's facing the screen? Meanwhile, Lyn's head is angled forwards quite a bit, so that she almost looks like she's looking at the ground ahead of her. She's looking so far away from the camera that she actually has an alternative sprite for when she faces the player.
Because their heads are such different angles, the Lyn's face and Lute's hair just...doesn't work. At all. Human heads are somewhat upside-down-sweatdrop-shaped, and the face has a very fixed position on said shape. This video might be about drawing, which you can't do while splicing, but it gives you a general idea of head anatomy so that you can easier spot issues yourself: How to Draw the Head from Any Angle
Character I: Again, another face/hair incompatibility. however, this miiiight be salvageable if you add hair to the far side of the head. Meanwhile... I'm sorry if this is rude, but are you colorblind, by any chance? Because the red shading on her shirt does not work AT ALL. You simply cannot shade bright red with gray-red. It just doesn't work. Period.
Character J: Her colors are all way too close together. They kind of blend in. Because of how dark her skin, hair, and clothing are, her eyes kind of stick out like they're glowing. But this can be ignored, in my opinion. The biggest issue is her far eye. It's not being shaded by the hair at all, and it needs to be. I think the darker green shade needs to be slightly darker, too.
Character K: I've circled the eye here, but the problem again lies with the angles not matching. In fact, the armor, head and hair are all different angles. Go back and study their source sprites. You'll notice that Forde has a VERY angled body, which just isn't working with the not-angled head. The hair is also off angle. The eye I circled is blaringly out of place given the head angle, but tbh... I'd say try this one again with different bases.
Character L: Is she dead? Because those skin tones are so pale against the rest of her body that she looks dead. Or deathly ill. Also, I've circled her hair in a very specific spot, but really, it's the whole hair that's out of place with the head. This is fixable, I believe... Hair is a limp object by nature. It's subject to gravity and tilts with the head. There are instances where it can stick out (my hair likes to floof sometimes, and I have several cowlicks) but in this instance, her hair needs to be tilted with her head. Remember, gravity is a factor.
Character M: Um... I want to say try again with the eyes, and don't reduce the shading this time. Also, for the parts of her hair that are colored differently, please go look at Heath. See how the lighter parts of his hair has its darkest shade as an 'outline' of sorts against the green? You need to do the same.
Character N: Mmm... I'm going to give this a passing mark and move on to O.
Character O: She has a very, very deformed head. Looks like part of it caved in or something. This is caused by, yet again, another drastic hair-and-face angle difference.
Character P: The circlet does not fit the head angle. Also, please don't mix and match FE8 head/hairs. FE8 has a lot of characters with really awkward head angles and it just...doesn't work.
Character Q: He has the creepiest face I've ever seen, but there's nothing specifically wrong with it (that I can tell.) The shading above his far eye needs to be adjusted to accommodate the round shape of a head, and the light source. Otherwise, good job.
Character R: I get that this is an eye patch, but what is keeping it up? It just seems to end right where I circled it. It doesn't look like it's going under/through the hair at all. I suggest splicing some bangs over it or something.
Character S: Mmm... The skin shading looks pillowed, but after all the critique I've received over the years, I have learned to avoid this even when it would actually be necessary to exist. So...I can't really help you there. The angles are great, though.
Character T: Another hair/face angle incompatibility making it look like she has a deformed head.