He initially worked in minor roles on several games after joining Atlus and took over as series art director from his mentor Kazuma Kaneko with Persona 3 in 2006.
As his family went through further moves while he was attending first-grade elementary school, he found the frequent changes in his environment unsettling, and became slightly introverted due to not being able to make friends.
Eventually, he made some friends in elementary school, who shared his enthusiasm for Doraemon and encouraged him in his pursuit of a career as a cartoonist.
He acted as sub-character designer for Devil Summoner: Soul Hackers, the Persona 2 games Innocent Sin and Eternal Punishment, and was involved in graphics work on Shin Megami Tensei: Nocturne.
His first job as art director and character designer was on Atlus' PlayStation 2 strategy game Stella Deus: The Gate of Eternity.
Having eagerly pursued the job after hearing about it from rumors within the company, he was initially overwhelmed as he had a large amount of work to do on the title and had yet to refine his drawing style.
[5][6] Several influences on Soejima's design and career were the work of Fujiko Fujio, the manga and anime franchise Patlabor, and video games such as Street Fighter II.
A crucial part of his design technique was looking at what made a character stand out, then adjusting those features so they remained recognizable even with the redesign.
[9][7] During his work on the Persona series, Soejima has used key colors to help illustrate the games' aesthetics and themes.