Julian was a Martian Gold of the house Bellona and son of Tiberius and Julia. When he introduces himself during their trip to the Institute, Darrow marks him as the kindest of the Bellona. It was Julian who introduced Darrow to his twin brother, Cassius, for whom Julian had only brotherly affection and not a hint of jealousy, and with whom Darrow would share a friendship (albeit short-lived) akin to brotherhood.
In The Passage (the final test before entering the Institute), Darrow is forced to kill Julian in single combat, a deed for which Darrow long feels terrible remorse and which (upon discovery) eventually leads to his alienation from Cassius and by extension, all of the Bellona.
During their Passage, Julian was somewhat insistent that Darrow (the scion of a lesser house) ought to simply allow himself to be killed. Even as Darrow watched the light leave his friend's eyes, he saw only confusion and shock at the circumstances under which a social inferior would have the audacity to take the life of a son of Bellona.
"A slender boy with an open gaze and pretty smile" ~Red Rising (page 117)