[citation needed] He was born as the son of the first marriage of knight Ulf Abjörnson of Engso, justiciar of Tiohärad (d. 1347) with Kristina Sigmundsdotter of the family of Tre Klöverblad.
Through his paternal grandmother, Karl was a descendant of the Ulv branch of the House of Bjälbo (Folkungaätten) and numbered some ancient Swedish earls among his ancestors.
His step-mother Märta Sunadotter of Hultboda, heiress of Fogelvik, was sister of Erengisle Sunesson, Earl of Orkney.
Helena was sister of Karl' first cousin Philip Nielson of Salsta's wife Ramborg Israelsdotter of Finsta.
1389, Karl inherited from his half-sister, Kristina Ulfsdotter, heiress of Fogelvik, widow of lord Peder Ribbing and of Niels of Rickeby, heiress of her sister Duchess Ingeborg of Halland and Finland, and her mother Märta Sunadotter, a daughter of Sune Jonsson, justiciar of Tiohärad, one of the first margraves of Viborg.
Karl was survived by his daughter Margareta Karlsdotter of Tofta (c 1380s - 1429), who was married twice; first to Knut Tordsson Bonde of Penningby and secondly in 1414 to Sten Turesson, Lord of Vik and Örby.
Margaret was the mother of King Karl Knutsson (1408–1470) and of Birgitta Stensdotter of Vik, heiress of Örby and Ekholmen.
Her descendants included Regent Sten Sture the Elder and Birgitta Gustavsdotter of Revsnes, who became grandmother of Gustav Vasa.