Dr. Jonasson is an ER doctor at Sahlgrenska hospital. He literally saves Salander's life by removing a bullet from her brain. He also makes sure she's as safe and as comfy as possible during her stay, shielding her from Teleborian and other malcontents. Salander can inspire such loyalty in people! Before you know it, Blomkvist has Jonasson illegally sneaking Salander her Palm Tungsten T3.
Jonasson treats Salander with care, which is a huge change from the treatment she received at St. Stefan's psychiatric clinic as a child. His trust in her, and his trustworthiness, probably help Salander's overall view of humanity quite a bit. One of the best things about Jonasson is that he's a foil for Teleborian. He shows us the way a doctor should treat a patient.