A Kree fanatic who was hired to fetch a stone for Thanos and decided halfway through that he would rather keep it.
Ronan is a fanatic whose family died in the Kree-Nova war, which the films mention and then decline to explore. What he is for, in practice, is scale: he is the thing standing between the Guardians and each other, and the reason four people who cannot stand one another end up holding hands.
He is also the catalogue’s first demonstration of what an Infinity Stone does to somebody unprepared, which is the lesson Infinity War spends three hours collecting on.