units who are 5 star locked and need refines?

Julia and Deirdre should get the nullify adaptive damage effect as their refine.

Also, maybe Shiro and Siegbert could use one?

A bunch of hem will drop to 4 to 5 stars early next year and then probably get refines later in the year

I think DC weapons need a refine in general.
Most of em are falling behind quite a bit.
And the one who doesn't is BK/Zelgius who often is used without it.

As for 5* units without unique weapon yet:
Luke, Mist, Gray, Faye and Genny