Well, the simple answer would be Daeron II, who protected him from childhood, gave him royal favor, and who he fought for at Redgrass at the peril of his own soul.
The more interesting answer would be Daemon, who he slew for the good of the realm.
Has to be Daeron the Good. Daeron was an intellectual and there is no way Bloodraven wasn’t. Especially if he hung out with Shiera Seastar.
Also Bloodraven killed Daemon Blackfyre, Aegon Blackfyre, Aemon Blackfyre, Haegon Blackfyre Aenys Blackfyre and torpedoed the majority of the Blackfyre rebellions both individually and as a whole.
It seems more like the brother he hated was Daemon.
That’s more straightforward, but kind of misses the “human heart at was with itself.”