Alden

Alden is the Fire Emblem's equivalent to this game's "Jagen" archetype if the Fire Emblem game we're talking about is Sacred Stones and the Jagen is Seth. Alden starts out really strong and will carry your team until you find a replacement - if you even do. While he does tend to fall off as a unit overall mid to late game, his roll as a permanent Repulsor Shield or Ancient Writ buff bot is always super helpful and a niche he has access to. 

Necromancer - In demonic, enemies' stats are inflated. Summoning is pretty strong due to being able to add a unit to take the aggression. With Scholar dip and Sanctified Ring, Alden becomes somewhat of a mixed tank having decent speed and bulk while being able to give mana, warp allies and add new ones while replenishing mana pretty reliably. 

Loremaster - The Loremaster is the worst class for skills; however, its passive makes up for it. I tend to like Aegis Loremaster, as it buffs my allies with Repulsor Shield and then reduces their mana cost by a small bit. When you need him to, Alden can take a hit and in Loremaster, he's generally fast enough that his personal passive (and personal skill if you used it) can put in some work. 

Cosmician - Cosmician is overall a pretty decent class to make Alden viable into the late game. Its just that other people do it better. The Inferno and Heir Rings put in some work; though, he would like a few more levels and some HP Elixirs to get the extra Mastery. You can go Warlock, Scholar or Aegis for the tier 2 class, but a 30ish reflect from a single Repulsor Shield is really good so I like Aegis.

Relic Knight - In demonic, Alden's bulk doesn't stand up to the enemies' attack power that well. That being said, you can definitely try and use him to tank a few hits here and there. Other great rings are the Love, Karmic, Plague, Victorious, and Chosen. Dimming Ring adds a lot of defensive bulk as well but will make his skills pretty bad.