[Great thread!
Personally, I want to see Lalli unpacked a bit more first.
We don't know much about his back story except that he and Tuuri apparently came along when Onni joined the army at Keuruu 11 years earlier. At that point, they would have been 8, 10, and 16 respectively. Onni and Tuuri don't seem to have parents or any other close relatives (or he wouldn't have accused Taru of "taking his whole family"). I'm guessing the lack of a family is *why* they came to Keuruu (and why there was a catch in Tuuri's voice when she told Emil about her childhood home).
What happened to Lalli's parents, we don't know. If they were alive somewhere else, Tuuri would probably have mentioned them at some point.
Lalli actually seems the most content with his pre-Expedition life; he only came along because Tuuri told him to. (As opposed to being motivated by loot and glory, the Call to Adventure, or the chance to meet tall, pretty, golden-haired Swedes.)
He is extremely inner-directed, neither as fearful as Onni or optimistic as Tuuri. Silent and watchful, he's almost expressionless within his comfort zone (and it's pretty hilarious that his comfort zone DOES include troll attacks but DOESN'T include baggage claim at the Mora decontam station, or horses).
In fact, Lalli seems almost emotionless except for rare moments, like when he comforts Tuuri on page 78 or reacts to her threat to leave him at the Sveavagen station, p. 147. (I don't know whether the Kuutar spell's "lamentations full of anguish" are literary convention or his actual feelings, but he's clearly not just reciting it like the rosary.)
Given that Lalli doesn't seem to have much of an agenda going in and is well-equipped to survive in the wild, he hasn't been set up plot-wise for disappointment or being hoist by his own petard. (Emil, now, has already set himself up to be taken down a few notches [*cough*girly scream*cough*].)
Everyone else in-story has agendas for Lalli, though. (Tuuri: get him better-socialized. Emil: befriend the poor mute Finnish lad, who clearly has NO IDEA how to take care of himself. The Adults: "See that enemy outpost over there, soldier? Go capture it.") The clash of their intentions against his inner compass will probably help fuel the plot.