Tatsumi was raised to be good at keyboard because he was an organist for his church as a young boy. He grew disillusioned with the church due to several incidents of widespread abuse within the institution. He then became a progressive priest and a keyboardist to reconcile his relationship with both religion and music and strove to change the institution and push it in a more modern direction, even as a teenager (come on, have some suspension of disbelied with the ages here). He entered Reimei and became dissatisfied with the system, disagreeing fundamentally with how the school was managed and governed, thus beginning the Reimei Revolution—details on the incident with HiMERU are in Aria’s doc.
His joined Alkaloid afterward and tried his best to forget about his past but guilt still chipped away at him, hence his need to apologise to HiMERU. His parents were displeased with his messy rebellious-to-kind nature and how he’d rebelled both against Reimei and their wishes (as they supported the Church), so they were cut off from one another the year Tatsumi joined Alkaloid.