Distant Counter options

IMO the best candidates are Effie,Eirika,Joshua, Lloyd and Marisa

I recommend to go with Effie or Eirika
Go with effie cause she will kill most enemies when she is fully built. and she's easily mergeable
go with Eirika if you just want her to counter attack at any range and buff allies and she's easily mergeable

Seth and Narcian are(from stats alone) not good for DC since they have pretty low res and don't that high of atk stat that they can kill enemies on counter attack.

Nowi has Lightning breath as an option so it's a waste on her at the moment.
if she gets a refine that does not have Innate DC then give her DC then.

I would say eirika because of her ability to recieve the buffs she gives out herself, which you can use to boost her attack. And if you want high scores in arena having dc on high merge units is a great way to increase your score. Effie is a pretty good choice too. Imo anybody with very high atk or good mixed bulk is generally a good choice for dc (like joshua if you really want to invest into him) but not on dragons like nowi, because you can run lightning breath instead and free the a slot for steady/warding breath.