The Sensei was once one of Ranith's employers, the Neutrals. But he'd found himself sympathising too much with people in the right and ran to his temple, raising all kinds of wards. So far, because he hadn't done anything against their cause, the employers were willing to let him live, but had Ranith on perminant standby to assassinate him. Too bad Secret Blade, viewing him rightly as the most powerful 'good guy' on their side so far, got there first.
He viewed Reion as his own son, and, knowing he was living on borrowed time, trained him to take over the temple.