Kagero or Lucius

Are those your only 2 options? If so, go with Kagero for what little anti-infantry she can still bring. Otherwise, I’d say hold your feathers for something better.

Depend of what your roster need, but I'd says to save your feather too unless you really love those characters,Kagero used to be a queen in arena, but became irrelevant with that Horse/Dragon/Armored meta,she is still good but even with +atk she has trouble killing tanky infantry,especially dragon.Lucius, welp,he is a good magical tank that can inflict pain,but he can pretty much do the same less effectively at 4 star.
Don't forget that Lyon GHB is coming,you might want to upgrade him instead of those 2 others units depending of how good he is.