@Betony
I think it's all family connections with someone with power or money.
Semimaru was probably the illegitimate child of someone rich/powerful, for Natsu I think it was one of her parent's cousins (not sure), Arashi and Hana is known, Hotaru because she and her sister were the next generation of Miko's of a powerful group, Matsuri and Chimaki I have no clue.
Remember that they were only accepting of people of certain ages, the powerful families probably had to pick distant relatives just because no one more closely related matched all the requirements.
Also, with team summer B, they themselves started wondering if maybe they were chosen for being people that had messed up somewhere in their lives.
Natsu dropped out of school and became a shut-in.
Semimaru was bullied and became a bit wild later on.
Arashi was violent for a period of his life before he met Hana.
Hotaru maybe competed with her sister for the right to succeed the position and lost? (guessing)
Matsuri was part of a girl group where she went along with whatever, even things she didn't like, just to fit in and be part of a group.
Chimaki, I don't know, but if he was like he is now, he probably focused on his art so much that he didn't pay enough attention to other important things.
The policewoman, I don't know. Best guess is her career hit a dead end?