So I'm about to start Mystery of the Emblem soon and I was wondering what I should expect and any advice going in as this is my first trip into Archanea. The only thing I know is that some units gain bonuses if next to others like Phina getting boosts from Navarre. You'd think I'd be playing New Mystery to celebrate the 10th anniversary of that game but I wanted to try out the original first.
Getting the true ending requires you grabbing all the Starspheres (named after the zodiac signs), as well as the other Sphere items. If you miss a single one, the game ends at you defeating Hardin. So be prepared to hunt enemies and open chests finding the stuff.
Every Starsphere provides stat growth changes for specific stats - both increasing the rate and decreasing the rate, and you can stack multiple spheres together by keeping them in a unit's item inventory, and every rate will be added together. Since every stat apart from HP in FE3 caps at 20, it is possible to cap many units' stats in quick fashion by trading spheres around. Getting all the spheres together is necessary to beat the game, but they all combine to become a single sphere some ways into the game, so they're not around forever, so get the levels you want quick.
Because nearly every stat caps at 20, mobility and Weapon Level rule the day. Magic users and Pegasus Knights will be your best friend. You get Palla in Chapter 3, and she's just about the best character in the game because she starts with killer bases. Palla, Catria, Caeda, Marth, Merric, Sirius, they're all bound to be all-stars.