Seiki Kagirinai

Important Information
Seiki initially shows a passive and laid-back personality, inheriting such traits from her relaxed life. When she is told to kill and eat as many people and plants as possible, because that is the zombies only food source, she attempts to. But she always runs away.

After the Fifth Impact, the Nuclear Zombie Apocalypse started, basically.

Seiki shows a great fear of emotional pain and of being hated or left behind, likely due to her perception of being abandoned in her youth and, subsequently, blaming all of humanity for this, primarily because the zombies devoured her entire family. Because they've started the Nuclear Apocalypse. In general, Seiki is shown to be introverted and disconnected from physical reality due to having little to no meaningful contact with other zombies, because of her introverted nature. In social party situations, she is shown to be quite uncomfortable, and she often has difficulty controlling his emotions around others due to his hard time letting go of the past. She is shown to the extreme of this, forsaking many lives (even her own) just to get her food delivered by someone else.

Due to Seiki going through all this, she has became a tsundere, and isn't afraid to show violence. Of course, like all tsunderes, she has a softer side to only those she loves.

Afterwards, she becomes a mercenary for Zomboss. If she ever existed in Plants Vs Zombies, she would be the hardest character to get rid of, especially if she had multiple versions of herself, considering she can change both humans and plants, to somehow become undead.

She has no known weaknesses, and is effectively impossible to kill. In her creator's headcannon, she can spawn multiple versions of herself, and stop time for 15 seconds in-game, meaning she can take out a few plants in those 15 seconds.

Although she's immortal and appears anytime she can, you can still beat her. Just, not a guarantee she'd stay away from Crazy Dave. You'd need lots of the upgraded plants (Lots of Gatling peas and Spikerocks can convince her to stray away for a little while)