You didn't like Shadow Dragon's writing? The story itself is simple, considering it's a remake of an NES game, but the actual writing I thought was really, really good. That and the game definitely had a few stand out characters. Shadow Dragon's biggest issue came from the lack of support conversations, in an attempt to stay more true to the original, but certain characters really suffered from this. Instead, they opted to more fully flesh out a choice few characters, but those characters I'd easily say are among my favorites in the entire series. In addition to Marth, who you mentioned, Caeda, Merric, and Minerva get tons of development due to the fact that they get a lot more conversations than most other characters in the game. I also like that Shadow Dragon made Cain an actual character instead of making him Red Cavalier #1.
New Mystery of the Emblem also helped flesh out a lot of the other characters, but as a game I tend to prefer Shadow Dragon over it. Both are great though.
Certain characters in Awakening are certainly one-dimensional (Kellam pretty much exists solely for his joke, for example), but quite a few of the characters this gets directed toward definitely don't deserve it. In particular a lot of the first generation units have a lot more depth to them than some give them credit for. Outside of the main trio, Frederick, Virion, Lissa, Sully, Stahl, Sumia, Olivia, Lon'qu, Libra, Gregor, Maribelle, Cordelia, Donnel, Panne, and Gaius all have quite a bit going on beneath the surface that I feel many people don't give them credit for, off the top of my head. I'd even go as far as to say that Tharja's Robin supports are some of her only bad ones, and that when she's not in her "obsessing over Robin" mode, she's actually a decently interesting character.