Selena

This is my "Quadlena" build, a Selena with the power and speed to attack 4 times to do great damage. For this sort of build I would suggest to +10 her so she can be at her absolute best.

A mixed-tank build for Selena. While her ATK is straight from a hot dumpster fire, Selena has very good SPD as well as decently well balanced HP/DEF/RES, which allows her to take on enemies across multiple different scenarios. This build does fairly well set into an Arena Defense team, as Selena can take on multiple enemies and do minor damage, which is sometimes all that is required for another of your defense units to finish the job. Unlike most top tier sword users, Selena is 3-4 stars and therefore easily mergeable.

---A SPD boon is taken to avoid doubles like the plague.
---An ATK bane is taken because, lets face it, she ain't gon do shiiiiiiittt, ni**aaaaaa (if you don't understand the quote please research Kevin Hart). Quick note, Selena does have an ATK super boon.
---HP isn't entirely bad because it can be fixed slightly with refinemnt.

---Wo Dao is taken to boost the damage she ain't gon do.
RES refinement makes it equal to her DEF, and allows her to run ploy skills decently well. A DEF refine can be take for her to target specifically physical threats, especially if you don't run Distant Counter.

---Aether allows her to do some actual damage while also giving her a little bit o' HP. It's also boosted by Wo Dao and, enemy HP permitting, guarentees 5 HP back. The point of this ins't to completely replenish her HP, but rather to allow her to last a little bit longer through more sets of combat.

---Reposition because thats why. Also its stock so yeah. Dual rally can be used for arena score.

---Fury goes for the A-slot in order to increase her defenses more. While the -6HP clashes with being a tank, it increases all of her stats, with no single stat being entirely throw away (even having bootycheeks for ATK is still important). You can also run Distant Counter, but this hurts her overall tank.
---Swordbreaker for B-slot, as the vast majority of units fast enough to keep up with Selena are swords, and also allows for the cancelling out of Bold/Vengeful fighter skills on sword armors like Black Knight/Zelgius, so long as she's faster.
---Like always, C-slut is felxible, and can be basically anything you want for the right price, big boii. Currently in is Attack Ploy, which increases her damage soak so long as she's able to get it applied.
---Sacred Seal is Close Defense, once again to increase damage soak.

Some other stuff that you can use on Selena:
--ATK is actually a SUPAH boon, so have fun with that I guess... probably go with an HP bane if that's the case? Idfk figure it out yourself, she's still gonna hit softer than a man with erectile disfunction.
--Safeguard w/ RES refine can be fun to increase tanking. Use Distant Defense/Deflect Magic seal to increase her longevity against mages. Use Close Defense seal for melee battles, where she can at least do some damage
--Steady/Warding stance can be used for enemy phase specific builds, and works great with Aether (also may want to look into Slaying Sword). However, this uses up the A-slot which relies heavily on the Fury buff, and both are increadibly hard to come by. Stick with a RES refine if you want magical soak/you use Distant Counter.
--Fury has good synergy with Vantage/Desperation in the B-slot if you don't like breaker skills.
--Renewal increases tankiness, and allows for a greater damaging special to be used like bonfire/iceberg. As with Aether, the end goal isn't total HP replenishing, but increasing just enough to extend her survivability.
--Guard prevents kill potential from specials, which are the best way to bypass a tank. This works much better with Distant Counter than it does with Fury.
--Dual Speed Ploy/Attack Ploy seal can be run, but more often than not the decreased speed is wasted on Selena herself, and is more for her fellow teammates.
--Defense Ploy/(offensive) seal can be used to buff Selena's measily kill potential.

Lv. 40 5* S Support +10 Selena that I spent a few months building. Ally Supported with Ephraim, can tank virtually anything that isn't Reinhardt and proc Ignis almost every combat via Steady Breath. Once she hits the Wrath threshold Ignis is a OHKO on nearly every unit in the game even if she'd be dealing 0 damage normally. Reposition is staying unless I can find more mileage out of Reciprocal Aid. Threaten Spd ensures doubles on similar-speed units and prevents them from Mia and Ayra. Distant Defense softens damage from mages and Brave Lyn while Close Defense synergizes can boost Ignis damage on Enemy Phase.

Jack of all trades. A Selena who I plan to be my first +10. With both good def and res she will be able to fight and win against most units who do t have the advantage towards her. Summoner Support

Selena's low attack hinders her ability to dish out a good amount of damage, +atk is recommended as it's a superboon that provides +4 to her attack stat. Fury 3 increases her overall stats and makes her fast and tankier with her well-rounded defense line.