Aethelstan

You could do no pre-promoted units. So no Seth, Saleh, Innes etc.

Hard mode is the perfect FE experience imho. Even without grinding you should be able to use any character you recruit if you like them. Though some will take a decent amount of hand holding. Pair up is mostly not necessary on hard and honestly sometimes you're better off without it because it gives you more actions per turn. You absolutely need it on lunatic. The enemy stats will always be too high for single units to reliably handle unless you grind. Even Frederick isn't perfectly reliable in the early game if he's unpaired.

Gerome and Lucina is my favorite. Their personalities suit each other as Lucina is the only female child who doesn't annoy Gerome in some way. He clearly respects her. And it's adorable he gave Lucina her mask. Inigo/Morgan is also great. Owain/Noire is sweet. And they probably end up having the world craziest child eventually lol

I use the "John NESS" app in Android. It works great.