Dante
Commander Dante is the oldest living (non-Chaos, non-Primarch) space marine who is not in a dreadnought. He is the victor of countless campaigns and his deeds are indistinguishable from legend.
In the plot, Sanguinius made a prophecy that there will be some great battle that will kick the asses of all others and that there will be a great, golden warrior standing between the Emperor and the darkness. Dante thinks that this golden warrior is him because he has an ego so massive that if it was a physical object it would collapse under it's own weight and turn into a black hole from which nothing but douchebaggery may escape. In other words, it is about 10% the size of Rowboat Girlyman's.
He also worked with a Necron army to defeat a Tyranid hive fleet and is secretly gay for a Necron Lord named the Silent King (if you can be gay for a genderless robot, enjoy your Black Library).