On page 76, Tuuri cries to Onni, "Onni, please come with us! I don't want you to be all alone!"
When Onni forces himself onto the boat on page 451, there is no one to see him off or provide moral support.
Onni is a twenty-seven-year-old, extremely powerful mage who is presumably a valued defender of a beleaguered outpost. Why is he all alone? Why doesn't he have a significant other or even a best friend?
For that matter, Lalli is a nineteen-year-old immune mage, a night scout which has to be a respected occupation. Aren't there any young women who really want to have his babies? I would think with the threat they're under, young women would be willing to put up with a lot of eccentricities if they could guarantee their children wouldn't turn into trolls.
It seems to me that their grandmother's one mistake must have been so terrible that her surviving descendants, especially the mages, are practically pariahs. Whatever happened was so terrible that they weren't safe anymore in Saimaa so Onni took the children and fled to Keuruu. Even there, the story followed them and no one wanted anything to do with them.
Onni took the brunt of this because he was eldest, and possible because he was a teen-age mage when it happened and either failed to prevent it or possibly even made it worse in his desperate efforts to stop it. He was able to make a place for the three of them because he is such a powerful mage that an outpost under constant threat couldn't afford to turn him away. But they are there on sufferance, and he knows it. That's why, on page 390, he tells Lalli that "We're not allowed to make make mistakes, not under any circumstances." If they are driven out of Keuruu too, where will they go?
Tuuri, being an extrovert and non-mage, was shunned at first but less so than Onni, so she managed to make friends over the years -- maybe not close friends, but friends.
Lalli, who was only seven or eight when they arrived at Keuruu, and who was an introvert anyway, was shunned from the start and never managed to break out of it. He became a night scout as soon as he was old enough because that way he didn't have to interact with anyone except, occasionally, his cousins.
Tuuri's decision to drag Lalli along when she decided to get out of the stultifying and hostile environment of Keuruu was actually an attempt, however clumsy, to help him by putting him in an environment where he would be better accepted.
My problems with this reading are the following:
1. We have not seen anything so far that suggests that the family has any stigma attached to it - Tuuri has a government job, and both Onni and Lali are mage-soldiers. If they were pariah, I think, they would simply not be allowed to live in a major community. Mage or not, if there was a serious bias against them they would not be this high ranking.
2. We actually have the opposite thing suggested to us by the comic - Onni, despite his skill, limits himself to the maintaining of wards or whatever shield magic is involved. I think it's hinted that this, considering the nature of Finnish defensive practices - which involves long range night operation to intercept any threats far away from the settlements, could be construed as cowardice. People acknowledge Onni's skill and powers, but no one speaks highly of him per se.
3. The move to Keuruu can be more easily explain by the death of their parents. Since we don't know the circumstances yet, or even if they died pre or post move to Keuru. I would propose the following chronology.
- death of parents. Transfered to the next of kin - which would be grandma, who lives in Keuruu.
- Grandma trains Onni and Lalli (maybe). Grandma dies. Onni becomes legal guardian.
4. This is my speculation, but I think the worst thing that Grandma did was probably to die. She made a mistake during an expedition and it cost her her life (and possibly other people - heck, we could probably have a situation that mirrors Lalli's own blunder, which in turn prompts Onni's intervention). I think that the events were quite traumatic to Onni, which in turn would explain his anxieties about the silent world and his family. Which I think is completely legitimate because dang silent world u scary.
5. We don't know if Onni is all alone, actually. Sure he has no girlfriend, but I don't remember any instance in the comic that says that he has no comrades or friends. His military occupation probably kills any chance he has of having any social life - if they were a focus of the comic. It's a lot more inexplicable that Sigrun - daughter of two generals, high ranking herself, in a town where people feast in common, to not have any husband - nor does Mikkel have any excuse, being heir to property and being a great housekeeper (though he probably wants to avoid from going from putting diapers on younger siblings to putting diaper on his own children too quickly). Compared to them, Onni has a solid alibi, compounded by what is probably a cautious, suspicious, and introverted nature that is very slow to trust or open itself.
Mysteriously all our protagonists are bachelors.