Just about done with my first maddening playthrough. Most of my final team has/had their niche at one point. For skills everyone has canter because that shit is broken.
Alear + Corrin: Dragon Veins are amazing if you keep her in her base class . Her combat is weak though, but her utility is unmatched. Also she has pretty good mixed bulk, insane HP, and good avoid so she's quite tanky. Overall disappointed compared to other self inserts, but still very useful.
Chloe + Sigurd: Very useful early game. Flight and doubling. Gave her Micaiah for the early game for flying healing and she ended up powerleveling a bit with Great Sacrifice. Went griffin, but regretted is once Ivy and Hortensia joined. Fell off as the game went on as enemy bulk outpaced her offenses. She still contributed with a levin sword once able to go Wyvren.
Diamiant + Ike: A God during midgame. He was instrumental in holding any choke point as avoid tanking wasn't broken yet. Engaging with Ike for better defenses and Sol made him almost unkillable for a while. Dex becomes a problem later on as he caps at 22, but he can still backup with his tomahawk and Ike stuff.
Alcryst + Lyn: I expected to drop him once I got Fogado, but Astra + Luna was too funny to pass up. Bows are quite strong with this game with triggering chain attacks and a healthy amount of fliers. He's a good boss killer too with Luna and not alot of bosses having 1-2 range. One or two speed takers let's him double swordmaster. Lyn does a lot of the heavy lifting with her pretty wild kit.
Ivy + Celica: Will almost never double, but her magic is strong enough to 2 shot most physical units. Instantly became my only mage once she joined, as she was decently tanky. Her Dex and Speed are quite lacking late game, but she's a good dance target since her personal let's her clean up her misses or nuke something else. Probably the best user of secret books. Also can use lot staves in a pinch.
Hortensia + Micaiah: She does Staff stuff which is insane this game. Potentially not spending a use does wonders for me being more liberal with staff. Not much else to say.
Kagetsu + Marth: Broken bases, great growths and instant sealing to whatever class you want makes him your best unit. Once I got Marth back, he became pretty dodge tanky while still being able to take a few hits as a Hero. If he goes Wyvren, it's quite unfair.
Zelkov + Lucina: He was instrumental in holding choke points with fog. No one would target him, but allowed me to slowly chip away at the enemies with 1-2 ranged weapons and canter. Poison is also nice. Yunaka is better offensively, but I enjoy Zelkovs avoid more.
Rosado + Eirika : Inherited speed taker and he was just gross. Competed with Kagero for best unit depending on the map. Lunar Brace increases your damage so much its not funny. Even without, he has great stats and can still contribute with any other ring.
Seadal: Dancer who doesn't die if sneezed on. Like that a lot.
Pandero was my filler guy and he used staves and hit armors.