I honestly love Eldigan's character but it's pretty hard to appreciate him with the game alone. No matter the translation, FE4 only has a few snippets of dialog with Eldigan that give a considerably bare bones characterization of him. It basically comes to the point that he follows an evil king because he is a loyal knight blah blah.
What really sets him above the rest though is his relationship with Raquesis. Yes, I know incest, but i think this was one of the only, if not THE only time this strange trope was used well in FE. EldiexRaquesis is taken as a tragic love story that can never come to be, but it has a huge impact on both characters. It really reminds me of Siegmund and Sieglinde in Wagner's plays. The best place to have this story played out is the Oosawa manga for FE4. Even though it isn't necessarily canon, I still believe it does an AMAZING job showing all of the characters from the first gen.
With what little the game gives you up front, Eldigan is a very basic character who realizes his flaw too late, but when you start digging into the lore and looking at Raquesis more, his character stands out so well. Makes me love Ares that much more.