Which Units should I 5*?

Black Knight or Effie. They're the only Tier 1 units from the Inheritance Tier List.

meh what are rein's ivs' if theyre bad. (not +atk) dont 5* him unless u already have a 5* rein +atk for the merge. other than that BK or Effie are your best choices

If you haven't got one already, consider nino (+spd or not at all).

soren and Klein might be nice as well.

I can't say for sure what is best, but I'll make a more condensed selection for you

-Nino
-Black Knight
-Effie (best to also put Brave Lance+ on her, but that will cost you another 20k feathers)
-Reinhardt (if +atk)
-Klein (if +atk)

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