Disa (DEE-sah) is an old Scandinavian name.
Even if the author was Swedish, the name could be of another origin, like Italian (as
Delia,
Delfina). Or the author could have made it up, to give the heroine a real unique name. Or Delsa could be a nn, for
Diana Elisabeth etc.
"You sought a flower and found a fruit. You sought a spring and found a sea. You sought a woman and found a soul. You are disappointed."
"It does not become me to make myself smaller than I am." (
Edith Södergran 1891-1923)