In case you havent noticed, Tsukiko can be hit up to 5-times (regardless of the damage dealt to her) during enemy turn and still survive. Such a "penta-gut" mechanic relies on the following skills and passives

Inari's Protection

Chance to ignore fatal damage (100%) when HP is above 50% (max 1 time)

Second Keeper of Time

Auto-cast Cleansing Ritual at the start of a battle

Chance to ignore fatal damage (100%) when HP is above 1% (max 1 time)

Cleansing Ritual

Recover HP (80%) to caster

Inari's Protection gives her a HP lock at 50%. Second Keeper of Time gives her another HP lock at 1 HP. If she has reraise on her (either from her CD: Spirit Fire - Nine Tails or through Re-raise spell/skills), she can tank 3 hits and die. Upon being re-raised, Cleansing Ritual heals her immediately to above 50% to enable Inari's Protection and Second Keeper of Time again so she can tank another 2 hits without dying.

Here is a demonstration video of how it works in battle.

There has been talks about how Tsukiko trivializes the upcoming Scorn of the Wicked Moon trial due to her insane damage as a mage, rivaling Esther's level of damage. With this "penta-gut" passive, she also trivializes the survival aspect of the fight as well. It matters the most when Moon starts to randomly hit your units and the attacks cannot be provoked.

NOTE: Tsukiko cannot "tank" 5 hits (regardless of damage) in two consecutive enemy turns. If she is hit 5 times in one enemy turn, she can only tank maximum 3 hits (regardless of the damage) in the following enemy turn.