I originally thought Rulk was Doc Samson, but I now think that the Rulk is a part of the Hulk, the Hulks Hulk. Im thinking that the Red energy Stark hit Hulk with is what caused the Rulk to exist. Just like the Gamma Bomb brought the Hulk out of Banner, the Red Energy brought the Rulk out of the Hulk. So when Rulk told Hulk that Hulk left Rulk alone and with a broken heart, my theory is that the Red Hulk is kinda like all the bad/evil that was inside the Hulk. What Rulk means is that the good side of Hulk (Green) is why Hulk is alone.
People are gonna hate this but Hulk and Rulk are kinda like Fat Boo and Kid Boo from Dragonball Z. Both VERY powerful, Fat Boo is the good while Kid Boo is pure the bad.
Dumb Hulk - Fat Boo
WWHulk - Super Boo
Rulk - Kid/Evil Boo
I know most people hate bringing DBZ into the Comic Book Universe, but I only did this to jopefully help people understand what might be going on with the Hulk. Its actually a Good thing that Rulk is split off from Hulk, theorstically Hulk is now pure good all his truly evil tendencies are gone and now manifested into the Rulk. The Rulk like the Hulk has been locked deep inside the Hulk brain, like Hulk was in Banner brain. When the Gamma hit Banner, Hulk came out, so when the Red Energy hit Hulk, the door in Hulks mind opened and the Rulk was free. Like When Fat Boo released all his evil and out came the gray skinny Evil Boo. Id see Rulk as either Evil Boo or Kid Boo. Evil Boo is the Evil side of Fat boo, but Kid Boo is the original Boo, before the Fat Boo existed and brought goodness into Boo. Maybe the Rulk always existed in Banners mind as did Hulk, maybe Banners mind as a child subcounciously created hulk as a way to lock up and keep the Rulk in check. So maybe the Rulk, like Kid Boo is the original Hulk and Hulk is the Fat Boo.