There is a list for the FE I played (I didn't bother with FE12 since there is only a few units more than FE11) of characters that are unloved or unused by most players (in my opinion).
FE 6 : Dorothy, Zealot, Wendy (We all wonder why) and Yodel (I think, I never saw anyone using him).
FE 7 : Wallace (not unloved, but almost never used), Lowen (?), Dorcas, Isadora, Renault.
FE 8 : I never really saw hated units, apart maybe for the Trainees. But I think it would be cool to see Rennac and Syrene in (real) action.
FE 9 : MAKALOOOV, Devdan (more unused than unloved), Bastian and Lucia, Largo, Ena and Naesala in the last chapter.
FE 10 : MAKALOOOV (bis), Meg, Fiona (if you use her you're my hero, she is really too bad), Lucia (AGAIN and she know why), Heather (but just because she's not that useful), Vika (idem), Lyre and Kyza (Laguz + They're between average and bad), and Ena again (who is here, but just for cheerleader usage).
FE 11 : Jagen, Wrys, Gordin, Bord, Darros, Rickard, Roshea, Vyland God, Wiendell, Bantu, Matthis, Jeorge, Maria, Midia, Dolph, Macellan, Tomas, Boah, Astram, Arran/Samson.