Ok, so I am all about this theorising on Onni's character and history.
The way I read their backstory so far:
> The Hotakainens mysterious grandma is possibly/probably Aino's unborn child from the prologue (or married Aino's child I guess).
> She lived in Saimaa with the rest of the family, her children were the parents of Lalli, Onni and Tuuri.
> These settlements were vulnerable to attack, everyone lived in fear of an outbreak.
> She was probably a mage and/or a night scout.
> She made some terrible mistake that lead to either a direct troll attack or an outbreak of the illness in their wee community.
Onni, Lalli and Tuuri fled this disaster, grandma and the rest of the family died either as a direct result of the mistake or in trying to fix it. Onni was older and a more proficient mage so he's both more aware of what happened and incredibly traumatised by it. They fled to Keuruu and its relative safety as a military base. During this escape he's suddenly responsible for the youngsters, and they're fleeing across the silent world to Keuruu. I can kinda understand him getting to the safety of Keuruu and just going "nopenopenopenopenope never going outside again here is good here is safe". Over the years this has turned into a mental habit of sorts and explain a little more of his total fear of the silent world.
It's interesting to note though: while Onni is terrified of going outside, and horrified by the idea that Tuuri is leaving, he doesn't seem so concerned about Lalli being a night scout and going outside regularly. He's very concerned about Lalli getting stuff wrong, but from what little we've seen he's not so fussed about the "going outside" bit.
Maybe he's just come to terms with it, or it's the awareness that it's a necessary evil, but considering how young Lalli would have been when they got to Keuruu I'd have thought he'd have dissuaded him from going into night-scouting as a profession. So...infection is a major source of his fear maybe? I mean he's scared as hell in the real world, but spirit world Onni is just sensibly wary of certain dangers. In the dreamscape/spirit world he's pretty forward and self confident, not exactly shying away from conflict/combat. As a non-immune mage he feels helpless in the real world where infection poses a threat, but in the spirit world he knows he can beat up anything that crosses him.
As for now though? He's actually left Keuruu, he's gone into the real world again. The question is though, is this just his fear for Tuuri and Lalli has trumped his fear for himself? Or has he taken a good long look at himself and realised that these two kids, who went through the exact same thing he did, are doing this and he's put them in danger by clinging to his safety blanket or never-going-outside? Like, if they can do this, why can't he?
