My first post, people.

Considering that Trevor and co. fought Dracula in the same battle in which Hector fled the castle, I suppose it's safe to say that Alucard knows Isaac and Hector. As far as I recall, the CD Drama says Alucard moved to the castle around time Lisa was killed (where he was joined by Lyudmil, his servant). Also, I don't think we officially know how long passed between Lisa's death and 1456, but Isaac and Hector were most likely recruited after Lisa died. And yeah, the CoD manga wiki article says Hector fled before the battle instead of in the middle of it, but still, pretty much the same idea.
So... headcanons about these three gentlemen.
Mine? Alucard, who was trained by his father in the dark arts, learned devil forgery skills. He might even have trained Isaac (who was not bananas at the time) and later on, Hector. I think he might have been closer to Hector than Isaac. They both had alchemist dads (who pretty much failed at parenting) and both were in the castle because they didn't fit elsewhere. Now. Isaac, I think, joined Dracula because he also wanted revenge against humanity (after all, Julia says she fled to the mountains because of witch hunts). Maybe Isaac's parents were burnt as witches? Maybe they WERE witches, after all, Julia is one, and Isaac has powers too. I suppose Mr. Laforeze tried to befriend Alucard because he identified himself with the young master, who lost his mother and home to witch hunts. But, since Alucard didn't seek revenge, and only stayed in the castle because he was young and confused, they didn't really get along after all. Finally, I think Hector might have been the final push Alucard needed to abandon his father's army. When Hector fled, Alucard realized he couldn't help his father anymore. He had already killed his only friend (CD drama says he didn't know Lyudmil had become a vampire) and Hector did what he had wanted to do since the start, thus, following his steps, he joined forces with Trevor and co.
Aaaand... yes. I wanna read your headcanons, folks.