Bartemius Crouch Jr.
A Death Eater around (and probably the same) age as Regulus.
In this comic
he's around 17 or 18.
During his younger school time Barty didn't have much time to make good friends, being to busy with studying to fulfill the expectations of his father. Seeming weird to his follow Slytherin students, because of his father and his high ambitions (Barty even chose Muggle Studies), and weird to the rest of the Hogwarts houses as well, due to his attitude, he finally got to know Regulus, and although they seem to be very different characters, they found out that they have in fact quite a lot of similarities.
They were both very clever, both came from old pure-blood families, they both loved Quidditch, and most of all, both of them were fascinated by the rising Voldemort and dreamt following him.
At the age of 17, both joined the Death Eaters.
Their relationship got less intense afterwards. This was mostly due to the fact that Barty, who former only had his friendship to cling on, now thought to have found a higher goal in life: To serve the Dark Lord as best as he could. He expected to get all the attention which his father never gave him now from Voldemort and soon came not only to get used to, but also to like the cruel methods the Death Eaters made use of to reach their goals. He even started to look down on Regulus a bit, who seemed not fully into everything they did.