As others have mentioned, there is probably not an exact formula for this, but i find that:
Character which are not noble swordfighters are usually more interesting, just due to the simple fact that we have so many of them in FE already.
The protagonists i most often see people like as protagonists are units like Ephraim (Noble, but lance), Ike (sword, but commoner), hector (noble, but axe), sigurd (sword noble, but horse), or lyn (sword commoner, but actually noble, and a woman).
Then you have protagonists which generally just have good dialogue, and feel human.
Holmes from Tear Ring Saga is a good example of this, and my favorite protagonist. Not only does he have a bow, he also doesn’t quite seem like a noble despite technically being one. He’s a flawed character, which makes him feel human and real, and he’s also fun to use, and strong gameplay wise. His character arc with Katri is quite adorable, and there’s been multiple scenes with him where i was just laughing out loud. Like the scene where he gets drunk and gets knocked unconcious by your jagen for talking crap.
Reese from Berwick Saga is another one i really like DESPITE being a noble sword lord. He feels quite human throughout the plot, and despite only being available for the main missions, and not the side missions (which constitute around 70% of the missions in the game), he still is better than most other fe protagonists. He’s shown as being of noble character inspite of being looked down upon by most other of the forces stationed alongside him in the capital. That however doesn’t mean he blindly help people without any regard for the big picture. One specifically memorable moment for me was when a handful of peasants asked for help against some particularly bad brigands, and he was quite hesitant to even accept the request because he frankly didn’t know whether he could feasibly do it with his small forces, which could be called into action by the king at any moment.
Having an interesting backstory also really helps i find. One protagonist that comes to mind for me here would be Roxelana from the Hack Drums of War (one of my absolute favorites btw, highly reccomend checking it out). She’s not your average young adult fe lord. She’s a bit older, a military veteran, with trauma from the past occasionally coming back to haunt her. You not only notice this throughout the story, but also due to the fact she’s a good bit higher level compared to the freshl, drafted recruits you get early on.
If all else fails, make the unit unique from a gameplay perspective. Micaiah for example gets a lot of shit by a good amount of people for story and gameplay related reasons, and some if not all of these concerns might very well be true, but at the end of the day, she’s a protagonist with magic, staves, and even a special sacrifice skill, making her more memorable than your average sword lord due to that alone.
Another protagonist who comes to mind here would be the main character from the hack Shackled power, who is essentially lyn with an axe.
An interesting gimmick could also help here. The best example that comes to mind for me here would be Arthas from Warcraft 3’s Frozen Throne expansion. While your other characters are leveling up like normal, Arthas, due to story reasons, instead levels down every mission, growing weaker. Thus you end up relying more on your other characters, while desperately trying to keep Arthas strong via stat boosters and items. This goes on all the way from being level 10/10 in the first mission, to level 2/10 in the final one.
Hopefully you can gain some insight from my ramblings. Had a slow day at work and needed something to spend my time with.