Agreed with @jinbatsu
If you’re fighting titans where you dont NEED to ascend Miki to Maximum to extend his survival it becomes a race between Frida & Vela (as miki doesn’t gain anything at 4/80 compared to 3/70 other than survival)
Between Frida & Vela:
Vela DoT & Direct Damage will both increase dramatically when you max her out.
Frida will only increase the direct damage which is the lesser part of her skill (compared to the debuff & elemental defence drop).
Both will only be eleigably for emblems at 4/80 but neither class looks like you have a maxed hero in them (Paladin or Druid) so not a massive factor.
If it was me, I would go with Vela over Frida because of how powerful DoT is at killing off enemies as it ignores any buffs, debuffs or any damage shares etc… & just kills them