Retirement

If a PC retires from play but stays in the village or the region, then turn them into an NPC. By default, their portrayal falls to you. But if everyone agrees, their original player can portray them.

Whoever portrays them, don't let them overshadow the PCs. They're retired! It's okay for them to advise and support the PCs, or to help meet the requirements of a plan. But they shouldn't act as followers or solve the PCs' problems. If need be, ask the ex-PC's original player why they can't or won't help with any given problem.