Undine

Undine is one of the Four Spirits whose attribute is water. She is calm, but prefers to be alone. She is also a close friend to Erubetie, and both are very alike. However, Undine views that coexistence between humans monsters is possible. Undine seems to have some rivalry with Salamander, having insulted the power of fire being “simple and shallow”. “Deeny” dislikes Sylph and traps her in water bubbles. She is the third spirit to be recruited by Luka.

When Erubetie attempts to use her Melt Storm in rapid succession, Undine interveines protects Luka with Aqua Pentagon having Luka. Erubetie knowing her best friend having a grudge on humans, she leaves. Having recruited Sylph and Gnome, she agrees to test him. When she asks that Luka uses both the wind and the earth, he does so, and passes her test.

Encyclopedia
''“A spirit who resides in a cave under “Undine's Spring”, out of view of the rest of the world. A long time ago it seems as though she appeared before humans, but now she’s a shut-in.

Hating the humans who pollute the waters of the world, she mercilessly preys on any who dare enter her terrain. Absorbing them into her body, she assimilates their body to become one with hers.

Though she dislikes humans, she seems to hold regrets that humans and monsters aren't existing in peace as they did in the past. A close friend of the Queen Slime, Erubetie, they treat each other as equals. They also seem not to interfere with each other’s dealings.”''

Strategy
Unlike the other spirits, Undine hits extremely high. Immediately use Gnome, then Death Sword Chaos Star. She'll use Aqua Prison, so struggle twice to break free. Attack a few more times until she spends a turn telling Luka to use both the power of wind and earth simultaneously. With that, use Sylph. Both powers will be active together, and with that, Undine agrees to join Luka.

Water Wall
At level 1, Undine's power uses to 2 SP to summon a thin wall of water around Luka, however it reduces damage by a very small amount (~10); far less than Gnome's power. Useless, it should not be used at all.

Serene Mind
Having being bitchslapped repeatedly by Alice, having being pressured by a Dragon Girl, and lectured by Salamander, Luka entrusts his mind and heart to the flow of the universe. At level 2, attacks taken will invoke Serene Movement, in which Luka moves like water flowing around obstacles and evades most attacks at a 100% rate (however Granberia (3 and 4) and Alma Elma (3) can still hit him). Attacking invokes Blade of Still Water, letting his blade follow the flow and increases the damage of normal attacks, however it does not score a critical hit. However, this technique requires massive concentration and uses 1 SP per turn and cancels when he runs out. Luka can use Serene Demon Sword to cancel Serene Mind before he runs out of SP. One of the most deadliest strategies is to use Serene Mind, then Serene Demon Sword, using only 3 SP to deal more damage than Earth Rumbling Decapitation, then fall back on Sylph + Gnome + Earth Rumbling Decapitation when SP runs low.

When Luka is pushed to the edge during Granberia's fourth battle, his determination further enhances the Serene Mind. At level 3, the power of water becomes compatible with the other elements, and uses 4 SP instead of ALL. However, the dodge rate will be significantly reduced.

Trivia

 * Despite having higher combat experience than Sylph or Gnome, she has the fewest amount of attacks compared to the other Four Spirits.
 * Undine mentions that Heinrich took at least one year to master her power.
 * Undine is the only member of the Four Spirits who kills Luka upon failure. According to Salamander, it is tradition for the Spirits to kill their defeated opponents, but the killing may be more related to Undine's grudge.