DC and Vengeful Fighter: who’s most suitable?

Triple distant defense and vengful fighter on jacob makes him take out almost every ranged unit so if you need that on your team it's a great choice, I do think his best build is with a brave bow and bold fighter though
Would say put DC on myrhh or reinhardt, depends on who has better ivs and who you'll use more I guess but they're both good options ( Myrrh can use lightning breath for a free distant counter though, and refining it for the 5hp and 4res/def makes it good competition with her natural weapon imo)

F.Corrin can have Lightning Breath+ instead if you want Distant counter on her, unless you like the thought of having Dark Breath+ with Distant Counter, her A slot is too valuable for the likes of Steady/Warding Breath.

Swordhardt isn't THAT mandatory to have distant counter, he can work well with his base kit as a player phase unit so it wouldn't hurt to just wait for another DC fodder to give him it.

Myrrh is better for DC, mainly because of the niche she brings to the table, having to stop any kinds of follow-ups assuming she has 5 defense higher than her opponent and can still make her own follow-ups with quick riposte.

Myrrh is a monster as she is but putting DC on her is the cherry on top. Her great def already prevents doubles and allows her to double, and being a flier allows her great movement options, along with great def/res stats she'd be my #1 pick for DC.