Being trans isn't a personality trait, it's just a human aspect. There's literally no need to have a reasoning for a character being trans, or a reasoning for a character being black, or anything like that, because people of all kinds exist everywhere. Even if they're only a half a percent of a given place's population, they still exist everywhere, and there doesn't need to be some special reason for them to exist. One of my best friends is trans, for example. There's no underlying reason he is, he just is. One of my other friends is black. I don't live in Africa, I live in North America, but he's not considered weird. He was born here, raised here, and is treated no differently. He's just a person who happens to be black. There's no need for a reason for a person to be a person.
Also, I noticed you accidentally said "she's" there? Caelum is a man, so it would be "he's".
"Being trans isn't a personality trait, it's just a human aspect."
I believe it to be a mental illness, this is why I called it a personality trait.
"Even if they're only a half a percent of a given place's population, they still exist everywhere".
There are also people who were struck by a lightning several times. Several times, Carl! But as people become of rare type and events become more improbable the whole story becomes less realistic (even if it DID or COULD happen in the real life. For example, it's totally possible to win lottery seven times in row without being a fraudster or buying all tickets. But it's very improbable). Don't get me wrong, I'm OK with unrealistic story if lack of realism leads to more interesting story (like story about a person who cries with tears of gold, for example), but it's not the case here. In "Beyond Yesterday's Grasp" I expected to get slice of life with a hero trying to overcome ghosts of the past. Instead I got story with REAL ghosts.