Krusniks look like normal humans when they don't activate the nanomachine, except that they age much, much slower than humans. When it is activated however, their eyes turn red, their voice gets deeper, they grow wings from their back, their hair becomes stiff, and they had a weapon equipped. Their wounds would immediately heal. Due to this scary look, people who see one's Krusnik form tend to call them as "monster". It is the last thing someone would like to hear. People normally don't like being alone. Everyone needs friends, and nobody wants to be alienated.

Seth grows a pair of fairy wings on her back, and wields two-handed gigantic tuning forks. When she transforms in front of Abel the squirrel, it is afraid of her, and when she turns back to normal, Abel refuses to go near her in spite of her invitation and smile. Her feelings hurt because of this—I'm sure it has happened a lot of times, but the feeling of being hated by someone who used to be close to you is just too hard to ignore.

"It looks like... Once again, I'm hated."

If Seth had a choice, maybe she would rather be a human or a methuselah. It can be seen when she gets in touch with Suleyman in her "spy" activities at the Island of Beloved Children.

Suleyman: I'm impressed—you can defeat Auto Lergars with those tiny arms of yours. Miss, are you a Methuselah?
Seth: Nope—I don't belong to such an outstanding race.

Seth looks wistful and sad while saying those words. It is actually a strange thing for her to say that Methuselah is outstanding—Methuselahs were humans injected with Krusnik nanomachines but fails to adjust with the Earth's atmosphere. Seth is one of the four babies who survived, so she should be proud for being perfect. But on the other hand, being immortal might be something that most people seek, but being "different" might be painful. I'm sure you'll feel lonely when the people around you die, while you don't. Then you meet new people again, and then they're gone as well. Not to mention, a "monster" is not something anyone would like to be.