Characters that break the Meta by either Great damage and Survivability, or by abilities.

As a veteran Fire Emblem player, I value the Speed stat above every other stats since it affects both your offensive and defensive capabilities. The only counters often to these builds would be enemies with both high damage and high accuracy outputs, but these enemies are often easy prioritize and remove from the battle. Once they're removed, it's just hoping your units pass the bar with enough stats and RNG to survive until the next turn.

Gywn
Nightblade> Champion/Ancarat

Important skill and weapon builds: You definitely want to build self-cast and extended duration Fleet Feet. Your weapon builds should prioritize either Shrink, Deadly, or anything that boosts your damage output without sacrificing any of your other stats.
Rings: Calamity Ring, Bloodied Ring, or Love Ring.

Summary: Gwyn's Ferocious Advance pairs best with Champion's passive Titanic and stats but I never found much usage for Champion's abilities. Her passive pairs equally as well with Reaper and Ancarant passive and abilities. Reaper is more utilitarian with mark and blink, while Ancarant is more survivable with glaive toss finisher and midnight ride.

Eve
Cuttthroat> Slayer

Important skill and weapon builds: Kris with Pure and Truestrike, alternatively any daggers with pure and deadly works fine too. Maxing out range for licky lasso is the way to go, helps with better positioning for your allies and for taking out key enemy units. I found Command Wyvern is the 2nd most used skill for her and imo any sort of combination works out, though you might find the lifesteal one the most useless as she essentially never gets hit if you build and play her right.
Rings: Depth Ring without a doubt. Solitude, Competitor, Blazing, or any rings that boost your damage capability is good.

Zanele
Tamer> Beastmaster

Important skills and weapon builds: Increase Gold and mastery scaling for Command Capuchin.
Shrink for suvivability and any other runes that increase damage output. As a beginner or even veteran player, having to amass gold to either rush builds or experiment around the game without having to worry about gold constraints is a godsend. This build will ensure kills but has no surviveability so having ally units to buff her survivability and rescue her is paramount.
Rings: Lava and Mystic Ring.

These units have either great utility, damage output, or surviveability but often not all 3 at once.

Riordan
Tamer>Beastmaster
Rings: Love, Bloodied, or Companion

Arthur
Geomancer>Cosmician
Rings: Surefire, Archmage, Charged, or Advantage

Cassandra
Ritualist then Waveseer>Tidecaller
Rings: Plague and Karmic (as ritualist), Sacred, Lifebinder, Prophet, or brightening

Haoran
Waveseer>Tidecaller
Rings: Sacred, Lifebinder, Prophet, or brightening

Tal'Dera
Conduit>Stormkeeper
Rings: Victorious and Soulfire

v1.0

Builds maybe subjected to changes as developers patch the game

This guide is unfinished and will be updated as the guide author has time to amend it.