Intriguing question!
Definitely agree with Roísín on rowan (Sorbus) for Reynir. Although an argument could be made for Capsella bursa-pastoris - shepherd's purse.
At first I thought Emil should be bunchberry, Cornus suecica specifically - suecica means Swedish. But then I realised he's got to be Epilobium/Chamerion angustifolium... fireweed!
Tempting to assign Trifolium repens to Mikkel, but I think not. Perhaps an Amelanchier, a serviceberry or as they call them here in Newfoundland, chucklypear. Like Mikkel, they are multi-purpose - early blooming for bees, tasty small fruit, tough and shrubby so a wonderful place for small birds to nest, a sort of jack-of-all-trades of the plant world.
For me, Lalli is always associated with Nuphar lutea, the yellow water lily, because of his dreamworld haven.
For some reason I think Sambucus racemosa, red-berried elder suits Sigrun, although I also like the iris suggestion! Maybe elder suits because it's a tough I-don't-give-sh!t plant, adaptable to changing circumstances, and very early to leaf out in spring... let me at it, it says!
I think Rubus chamaemorus, the cloudberry, suits Onni. It's understated and easily overlooked, rather solitary, but tough as nails.
Tuuri's the hardest. Violet is nice, but I finally settled on Campanula rotondifolia, the bluebell. Tuuri is eternally optimistic, and to me bluebells are too - I've found them growing in the darndest places, including on the rocky lip of a waterfall.