It’s the person not declaring themselves as the expert.
The person who works on the problem leads the team, stands up, and figures out what is going wrong and how to fix it.
They don’t declare themselves expert and would never dare to declare themselves as such.
But to everyone else on the team, they are the defacto expert and much more so than the one that is standing up to say “I’m the expert, I’m the master, ask me.”