Mori is a girl who never found where she wants to fit in. She couldn't hang with the girls, so she spent more time with guys, creating her tomboy streak. But eventually she got bullied out by them as well, socially isolating her and pushing her onto the internet and creating her interest in Japanese music and media. Her desire to be famous (grinding her music, wanting to collab with E-celebs, etc) is because she has a deep-seated desire to feel important because then she'll feel like she finally has a place to belong.
Kiara has deep scars from coping with stress for a long period of time. She set out to follow her dreams despite judgement from others and got the stuffing kicked out of her horribly from multiple sides, and the trauma has left her hyper sensitive despite trying to be the "tough bitch" she wants to be.
Ina realized how lonely it is as the top. She followed her dreams unyieldingly because that is supposed to be the recipe for success, but then she realized she was exhausted, burnt out and lonely with nobody to share her struggles with outside her family.
Gura's got the worst case of imposter syndrome. Growing up, she was almost certainly bullied for being small and spindly, which damaged her self esteem. Combined with naturally occurring anxiety and depression (evident in her ghosting habits/wildly , and her self image is horrible. This clouds her eyes from seeing her talent, and sees her successes as flukes of the algorithm, or as an elaborate joke where she's the punchline.
Amelia didn't have it good, and that's what she's learned to expect. In her own words, her family is cursed. But the human mind is great at coping, so even in the worst of times Amelia could keep her chin up, even if it caused her to develop some unhealthy coping mechanisms, like her stubborn attitude. But now that she's made it, she's still waiting for the other shoe to drop and it to be pulled out from underneath her. She's scared to let people close and scared to enjoy her success because she doesn't want the hurt of having it all taken away.