I think you (and later she) will probably always be explaining the spelling, but I imagine you already have to do that for [name_m]Kaspar[/name_m]? [name_f][/name_f] It pairs well with [name_m]Kaspar[/name_m] but I admit I prefer [name_f]Kaia[/name_f] and [name_f]Tuva[/name_f] to [name_f]Sylvi[/name_f] for you. [name_f][/name_f] All are lovely options, but I feel like I hear [name_f]Sylvie[/name_f] these days much more than [name_f]Kaia[/name_f], so [name_f]Kaia[/name_f] and [name_f]Tuva[/name_f] feel more fresh to me, and more naturally Scandinavian? [name_f][/name_f] [name_f]Sylvi[/name_f] feels a lot like [name_m]Tobias[/name_m] or [name_f]Anna[/name_f] etc., which could belong to 20 different ethnic origins? [name_f][/name_f] But the spelling is fun and so is the meaning, and I do like her with [name_m]Kaspar[/name_m]. [name_f][/name_f] I am obsessed with [name_m]Kaspar[/name_m] and [name_f]Kaia[/name_f], though, as I think you know by now, so that is probably also impacting my opinion 
Good luck!