Ramble ahead alert
I just started thinking and didn't stop
sorry
So I feel like even though Shyshie has a special connection to each of the Storm birds, after she frees them from the Ocean Princess's control she becomes actual friends with Scirocco. He's always been the one who was most interested in mortal life, in learning about even the more mundane things.
He was always fascinated by songs, stories, poetry, and the way changing the infections in a voice can change the entire meaning of a sentence, how the same words can mean love or hate simply by the tone. Being the keeper of wind he's always restless, and being trapped in a stone for years at a time only exacerbated his wanderlust. He relates to Shyshie different than his siblings do. She feels pulled between land and sea, selkie and mortal, and he constantly feels torn between his duties and the desire to travel, to abandon all semblance of responsibility, and just go.
He was interested in Shyshie from the beginning since she was different than all the Stormkeepers before her, being of mixed blood, far younger than any other, and a defiant attitude towards the Princess.
Since Shyshie was so young and reckless, Scirocco decided to self appoint himself as her confidant and counselor, trying to guide Shyshie in her newfound power and they quickly bonded more than the other two Storm Birds.
Tal and Astrapi both care about Shyshie's wellbeing, as its their duty, and they do like her, but but the friendship isn't as strong between them.
Partly because after being released from the Ocean Princess Tal and Astrapi were both able to choose for themselves how and where to spend their time and energy.
Tal hasn't really befriended any mortals, of all the siblings retaining the more aloof and distant. He prefers to spend his time alone, and is happiest that way.
Astrapi seems to delight in strong, stoic, and steadfast mortals, the apparent opposites of her personality. She has befriended the likes of Bella, Fern Rose, and Jasmine.
People who would be described as stiff or even humorless. Astrapi feels like they balance her out
and while they are often capable characters they are often overlooked, unfairly in Astrapi's opinion, so she enjoys the shocked attention it brings when she chooses to be at their sides.