The most commonly discussed criterion for the selection of someone for the role of a
project manager is their ability to be hard-nosed. When it comes to delivering transformation and improvement, I would strongly advise against using a project manager whose major tactic is to drive people in to submission through aggressive management.
It is clear that the hard-nosed approach can work but only when people have too many priorities (in which case select full-time people for the project) or when they are lazy (in which case do not select them at all). By definition, transformation and improvement projects are likely to involve covering new ground though innovation. An aggressive project manager will stifle this innovation. They meet beat people in to hitting deadlines but not in to delivering innovative solutions which will benefit the whole organisation.
When looking for a project manager for your improvement project, my recommendation is that you pick someone who can co-ordinate people to work together in a team to develop cross-functional solutions. They should be good at laying out a methodology which provides a clear direction and strong at rapidly solving problems as they arise rather than insisting that the team stick to the deadlines regardless. Drive is important in transformation projects but this is delivered through commitment to the end goal and teamworking.
So when it comes to picking a project manager for an improvement project pick someone who is a strong and methodical leader, who is good at instilling team working in cross-functional teams and who can solve problems when they occur rather than getting flustered and resorting to an aggressive approach.