sumireko usami is a 16 year old girl (as of Urban Legend in Limbo), she is the first president of the secret sealing club. she has psychic powers and visits gensokyo in her sleep.

she first appears in Touhou 14.5: Shinpiroku/Urban Legend in Limbo. using occult balls, she is able to enter gensokyo, though for limited amount of times.

she scatters said occult balls in gensokyo, then waits for a resident to collect them all.

when they do, they are sent to the outside world for a limited amount of time, but enough for sumireko to learn about them. soon enough, sumireko is met by mamizou futatsuiwa who claims to have gensokyo's own occult ball. which would allow sumireko limitless travel between the outside world and gensokyo.

after beating mamizou in a fight, she is able to claim the occult ball mamizou offered. but it seems that it made her enter gensokyo the normal way. trapping her there.

upon being trapped, her story mode begins with several residents trying to scare her as to teach her a lesson. by the end of her story, miko confronts her.

she reminds sumireko that she can use the occult balls to temporarily return to the outside world. when she does, she believes she'll be killed by youkai if she returns. she decides that she will sacrifice herself when she has to return (she doesnt)

after being brought back, she is confronted by reimu, who had realized one of sumireko's occult balls had been replaced with a lunar capital ball.

and if it were to be used, it would connect gensokyo to the lunar capital. after a fight with reimu, it ends up being activated. setting up LoLK. following that, sumireko is detained by reimu, mamizou, and kasen. she is later returned to the outside world upon deciding she's not a threat.