Huh? There are tons of characters in SW that just showed up with barely any lines and were instantly likable. Shin just continued the tradition, unlike some others...
She's hot. That is such a huge push but it's just not said very much.
Also helps that she's competent, doesn't complain, her story arc isn't built on her acting radical and/or like a rebellious teenager and she actually acts like an apprentice. A few scenes, you can see it in her eyes, she's not quite sure this is all going to work out. But she also understands she's not going to question Baylan in front of others, because as soon as they decide to kill him, they'll kill her too.
Hera, on the other hand, is plain exhausting. Her dialogue reads like a jaded and super angry Plenty Of Fish profile. And Sabine is written as a Peter Pettigrew. ( "I can't tell you anything even though people are trying to kill us both, it's so complicated, I also won't tell you that I'm love with you, the Jesus look is really you though") There are issues with plot armor for them, and it's impossible to assess that they have ever operated in some kind of actual chain of command system. Ashoka just stares at them from afar, like she's hopped up on Ritalin. Compared to them, Shin is just way more grounded. Probably helps that pretty much everyone loves Baylan, our lightsaber wielding Titus Pullo, so if he picked her, obviously he picked her for a reason and it's a good choice. We believe in him, so we believe in his choices.
Shin is no hairdresser, she's in the unicorn zone. Of course she's going to completely rattle and capture the male audience immediately.
Just some thoughts.