Jiro Yamashita transitioned from a chemistry teacher to an idol at 315 Production after meeting colleague Michio Hazama, who dreamed of becoming an idol. Initially hesitant about leaving teaching, Jiro embraced the career change upon recognizing the superior financial prospects of idol work, a practical motivation setting him apart from more idealistic peers.

Standing 185 cm tall, Jiro maintains a notably lax and often slobbish demeanor; colleagues observed his frequent neglect of appearance, though he could appear surprisingly polished when groomed. A recurring habit involved misusing school science equipment to brew coffee, drawing frequent safety-related reprimands from Michio.

Within the idol unit S.E.M., Jiro represented the "S" for Science alongside Michio (Education) and Rui Maita (Mathematics). Their first performance leveraged their educational backgrounds at a preparatory school. Early audition failures led Jiro to consider quitting until Michio expressed guilt over involving him and Rui. This prompted Jiro to recommit, acknowledging their former teaching careers lacked both financial security and excitement.

Despite professing a focus on wealth, Jiro displayed hidden sensitivity, becoming bashful when others acknowledged his kindness or emotional investment in the group. His former educator skills occasionally surfaced during idol activities, particularly in structured or mentoring roles. During a major typhoon threatening a Jupiter concert, Jiro joined other 315 Production idols as makeshift crew members to ensure the event's success.

Jiro adapted to collaborative environments like joint concerts and training camps, working alongside diverse idols. His journey balanced pragmatic beginnings with gradual emotional investment, while his core laid-back attitude and occasional rule disregard remained consistent.