Euclid:
We don't know all that much about Euclid.
He probably lived in Athens for a time and might have set up a school in Alexandria in what is now Egypt. He was a real person, not a myth!
We do know that Euclid wrote four books about Maths.
He's responsible for a lot of the maths you leant today about geometry. In particular, anything to do with triangles is probably based on ideas that Euclid wrote about.
He also came up with a way of working out the Highest Common Factor of two numbers.
In fact, he produced an algorithm to guarantee to work out the Highest Common Factor every single time.
And where there's an algorithm we can usually write a computer program...