Steady Breath candidates

I got four of them today, and put them on Hardin, Zelgius, and Y!Tiki.

Basically, you want it on units who have good defense because it requires getting hit, but also units who have a special that you want to activate quickly. In Zelgius' case, it gets to Black Luna quicker which is what you want because that kills everyone. It also works beautiful with specials like Aether because they have a long CD but are really damn good and useful in Arena.

Dragons are the easiest candidate for it. Nowi and A!Tiki make excellent use, and it's decent for Fae as well, though I have Warding Breath on her because I like her as a magic tank.

There are certainly other units, but just follow the idea that if they have a good special and can take a hit or two, that it will make them much better.

I think that you should save Steady Breath for a unit that you plan on max-merging for Arena. It's such a good and premium skill that I wouldn't just give it to any unit, even if they would make good use out of it.

Of the options that you list here, any of your Dragons with Lightning Breath+ would make fantastic use out of it (in tandem with Aether). I would primarily focus on A!Tiki, Nowi, or Fae because they are much easier to max merge than any of your other units.

If you don't care for Dragons, then Ike and Zelgius make fantastic use of it, though I would prefer to give it to Zelgius because he also has access to the Armor-exclusive Bold Fighter to become a monster on both phases. However, I would recommend against giving it to these guys unless you plan to +10 them (which is extremely difficult to pull off if you are not a whale).

A.Tiki vs Y.Tiki is a question of taste, merge potential, and, at least for me, whether you want to put guard on A.Tiki, which forces her to hog the QR seal. I prefer Y.Tiki because I generally prefer my tanks to be fast, but I also have an A.Tiki and her def is truly stellar, especially with steady breath. If you want to merge your Tiki a lot, A.Tiki is probably the better choice because she's not 5* exclusive.

For the others, the first question you need to ask yourself is which units you like best and which colours you want to solidify in your barracks. SB is an extremely powerful skill, so use it on a unit you like who is needed in your roster. If you're me (obviously you're not, but anyways,) you would have one red SB tank, so I would be looking to give it to a non-red. (Young Ike isn't that good of a unit anyways by my estimation; well eclipsed by L.Ike, who's also free.)

Nowi is an interesting case if you already gave her close def...that's not a way to build her that I would recommend, but it definitely has its merits. If you intend to abandon the close def route, she does very well with SB. Otherwise I'm thinking use bonfire and either guard+QR or QR+close/distant def, as you prefer. Distant def is probably better because A.Tiki is the best at walling physical attackers, but there's a lot of competition for this seal just as there is with QR.

Fae is kind of an awkward case to me. I like her chicken art, but that has nothing to do with her combat potential. What's awkward about her is that she starts with 25 def. She's the right colour dragon to check B.Lyn and Reinhardt, but her def is problematic because B.Lyn hits so hard and pretty much always doubles her unless you panic the horse and hone cavalry comes into effect. You need to make a decision, whether Fae tries to take on strong bows and daggers or not. If not, and somebody else is taking care of axes, I think warding breath + close def is actually a good option for Fae which enables her to leave the distant def and QR seals for others. 31 def will be enough against a lance, and she has 34 res and a lot of HP to deal with blue and green mages. However, if you want her to deal with any ranged threat that isn't red, I think she's forced to take steady breath + distant def unless you buff her speed so much that even B.Lyn can't double her. (One guy I asked uses Fae +spd with spd refine on lightning breath and fury, then ally support and buffs to get well into the 40s.) Unless you block the double, she will probably need a def buff, or you will have to use a +def weapon refine, in which case you may as well use a +def IV also.