Who to inherit Steady Breath?

Who do you use more? Who has more merges/is your fsvourite character or is part of your arena team? Consider all of that

All of the infantry you listed serve a similar purpose. Assuming no favoritism, I would say nowi or hardin or shiro

i would say nowie.
Easy to get merges on, Dragon BST, SBreath balaces out the backdraw of lightningbreath, adaptive damage of refined breath Weapons.
Nowie got my vote XD

I'd say Fjorm, if you ever plan on merging her. With blessings, she's an amazing score booster in arena as well as a guaranted bonus unit every so often. However if you don't plan on merging her, Hardin. He's an amazing mixed phase unit with it. Just make sure to give him the quick reposte seal.

Hardin Fjorm and Nowi are the best for it since they have inbuilt DC. It helps Fjorm most since her damage is low by default and her defense could also use a boost, but Hardin and Nowi both become absolutely busted with it.
If you prefer to use Shiro or Lukas (or Lancina or Neph in the same vein) however you can slap it on them. It depends on who you like best and use the most

This is a tough decision. If you combine Steady Breath with Aether on a character, it's extremely powerful.

I'd say Hardin is the top candidate because he also has Bold Fighter and if you give him Quick Riposte, he can be a fantastic anti melee, missile, and magic unit. Though, Hardin functions best as part of an armor emblem team. So, if you have 3 other distant counter armor units, you've got an arena powerhouse team right there.

Shiro would be the next candidate for consideration as the def boost would combo nicely with his Bright Naginata's effect of +4 def when attacked. And again, giving him Aether would also be a huge benefit. Fjorm would also have possibilities given her built in distant counter, but I would replace her Ice Mirror with Aether in that instance because she is a bit fragile.

If you like Nowi and use her a lot, she could work. Don't consider Lukas unless you are constantly using him. Good luck!

I agree with those who say put it on the character you will use most. Some things to consider:

Unless you want to build Nowi for player phase (and I don't believe it works well, but you have the IVs for the best try), I consider SB her optimal A-skill. I much prefer her with SB than with any other A-skill.

Hardin is better than Lukas unless you want to merge to +10 (almost certainly better than Shiro too unless you prize maneuverability), and Nowi is generally straight up better than Fjorm for several reasons, including thick HP pool, invulnerability to lancebreaker (which is usually a Reinhardt issue), adaptive damage vs ranged, refinable weapon, and merge potential on two fronts - 3*/4* availability, and Fjorm is actually good fodder while Nowi is relatively poor fodder.