Mozu if that counts. But she has plenty of time and easy ways to be trained up even in Conquest. So she prolly doesn’t fall into this category.
Then again, idk if Est herself from SoV qualifies as an Est because she doesn’t join very weak before the endgame but rather before the midgame.
I usually think about them as characters that join very weak but who can be trained up to become very strong. The potentially strongest of the game - But multiple people told me that the term Trainees would apply more to characters like Mozu, Amelia, etc. since those join very early compared to Est from FE1/Shadow Dragon.
If we’d be talking about the effort you have to take, Fiona would be the end of the iceberg. But no matter how much time you put into her, there are so much better units or units that are easier to train up and perfom better or the same.
Nino is the one that I like the most storywise. But unlike Delthea (which also kinda joins midgame instead of endgame if I think about it), Nino has multiple problems gameplaywise. You need specific promotion items. And hers is a guiding ring which has heavy competition. Also no matter how hard I try, she always ends up getting killed in one of my runs. If I train her that is. (Funnily in a current randomizer run of mine she really became a Pegasus Knight to transform to a true Est-Character). I know statistics, etc. might not be the right thing in this thread - But for me personally these are the big factors as to why I don’t like using her. In the end there is not really much that she can do better than Pent, Priscilla, Erk, Lucius, Canas, etc. But I’ll might have to use her in a future Ironman who knows. But I get why people love training her since she get’s so much Exp after every combat.
Training up Delthea is a lot more rewarding and fun imo since she will outclass her brother and mages are rarer in SoV. Plus you have no requirements except reaching a specific level. Training her is fun for me because it doesn’t really require extra work (except getting her recruited prolly I always forget how to do that). It feels like playing the game normally.
While we are at the topic of mage girl Ests. Taltiu doesn’t need to be trained to fulfill her biggest purpose: Giving birth to Arthur. Preferable with Lewyn. But she can be leveled up with the villagers of Chapter 4 pretty easily if you want to that I think. So technically this is the most effective Est-Character and being a unit that joins at the end of two chapters before Gen 1 finale she fits right in.
While I think about it. Would Corpul/Sharlow qualify as an Est too? His growths are high if he has Holy Blood. Staff Units can be trained kinda easily. And if Lewyn is his father you even have a ridiculously strong unit.
Also there is Sigrid from Vision Quest but I effed that one up because I couldn’t decide wether or not to use her and I promoted Hollace instead… But next time I’ll try her out, she has a brave light tome and get’s 1 level per kill so that should feel good.
But in the end I think I pick SoV Est if that counts. No matter if she fulfills the requirements or not - She has the name Est so she IS an Est-Character. And because of that, she is the best of them probably. At least imo.