Tamamo or Kiyo Lancer

From my experience, Kiyohime Lancer is a buster/buster crit servant with her kit focusing on increasing her buster damage.

Pros:
- EX mad enhancement (12% increase)
- Bath Transformation (can hit a max 30% increase for 3 turns)
- Pursuer of Love (decreases enemy defense by a max 30% for 3 turns with a demerit of 20% enemy attack increase)
- As for CE, I've been running her with a MLB Joint Recital (20% buster and 20% crit damage increase) totaling to more than 2x her base damage just on buffs alone

Cons:
- No survival tools (No evade, invincibility, def up or even heals)
- Crit debuff abililty is situational (Can be good against assassins and archers but not much else)
- Is good at one thing which is dealing buster damage but not much use anywhere else

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On the other hand, Tamamo Lancer is more well rounded:

Pros:
- Strong anti-male NP
- Good NP gain
- Good star generation
- Provide team wide support thru Beach Flower EX (similar effect to Charisma and has a bonus of additional crit star gen for males)
- Lots of survivability (Has charm with NP charge demerit, Has invincibility with stun demerit, HP is quite Tanky at 16K)

Cons:
- 2 out of 3 skills have significant demerits meaning you have to work around them (e.g. using charm when boss has max NP charge, using Atlas Mystic Code or bringing a debuff cleanser to clear stun debuff if boss isn't killed by Tamamo's NP)

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All in all, as much as I love my Kiyohime Lancer, I would recommend to go with Tamamo for her extra survivability and her boss killing particularly males. Just be careful with the demerits on her skills or bring a debuff cleanser.

Hope this helps!

I also pulled Kiyo lancer and tamamo lancer from the summer event and asked the exact same question. My replies were mostly that tamamo was better in general but I leveled up Kiyo first just cause I like her more.