Scrum master
What makes a good scrum master?
The coach, facilitator, impediment remover, and servant leader of the delivery team. The scrum master is one of the 3 roles on a delivery team. The scrum master provides process leadership and helps the delivery team and the rest of the organisation develop their own high-performance, organisation-specific Scrum approach.
https://www.innolution.com/resources/glossary
Geoff Watts author of Scrum Mastery From Good To Great Servant Leadership says the qualities that are always needed of a Scrum Master are:

Common challenges that can impede a SM from being effective:



Decision Maker – it is not down to the scrum masters to make decisions on how something should be done. They will ensure all the available information is presented in a way that can be understood so the PO can make the decision.
Go to person instead of coach – the scrum master although tasked to unblock issues should not be the first port of call, the team should have tried amongst themselves to solve the issue before raising to the SM. If the scrum master ends up being the go-to person the self-organising team that is agile is no longer working.