"Afton seemed like the normal one in the venture, the businessman. Henry was the artist; he always seemed to be off in another world..."
Before things took a turn for the worst, William and Henry were close friends and business partners. Some people believe William never truly cared about Henry and was only using him, but I believe they were real friends working together towards the same dream. Though we don't know much about William or Henry before they met each other, I believe they both found a source of kinship in each other as two men who are slighty off-kilter.
"A search of [William's] house had found a room crammed with boxes of mechanical parts and a musty yellow rabbit suit as well as stacks of journals full of raving paranoia, passages about Henry that ranged from wild jealousy to near worship."
In the books, William is intensely jealous of Henry, particuarly his genius when it came to robotics. He is so obsessed with Henry's creations that he spends three novels chasing Charlie down to retrieve a piece of Henry's machinery that William claims belongs to him. To William, Charlie is an extension of Henry; his talent, his personality, the creation he left behind.
"My old faithful partner, who I can only hope now is in a grave of his own, took what I had begun, and made something of his own--something dreadful."- Henry about William in his suicide letter (The Fourth Closet).
Despite their mutual animosity near the end of their relationship, it's clear William and Henry both cared for each other, or at the very least Henry did in a one-sided way. This is evident in the way he talks to William before both his game and novel suicides, calling William his "old friend" and "faithful partner." Henry never would have began a business venture with William if he didn't trust him; only after a terrible tragedy did he realize his trust was horribly misplaced.
Their emotional journey from good friends to enemies hell bent on the other's destruction is partly why I find this ship so appealing. Neither of them are good people. William is a remorseless child serial killer who took the life of his best friend's daughter for seemingly no reason. Both of them are terrible parents. Henry is, at best, an absent parent, and at worst, a neglectful one. In the games, Charlie dies because of Henry's lack of attention to her, either allowing her to wander off alone or, unfortunately, wrongfully trusting her with William. In the novels, Henry's delusions about Charlie ruin his life and his relationship with the other child he left behind, Sammy.
Fredbear & Spring Bonnie
At the original Fredbear's Family Diner, there were only two animatronics: Fredbear and Spring Bonnie. The animatronics doubled as wearable mascot-style suits. When being worn, the mechanical parts of the animatronics are pulled back by mechanisms known as "spring locks". Misuse of the suits or accidental discharging of the spring locks results in mutilation and a slow, bloody death.
Though the suits could be worn by any employee, the Fredbear suit was associated with Henry and the Spring Bonnie suit (which he used to lure and kill many children) was particularly associated with William.
There's a cruel irony in the fates associated with the suits tying into Henry and William's mutually destructive relationship; William used the Spring Bonnie suit to abduct and murder Charlie, Henry's daughter. In the games, William's youngest son is accidentally killed when he's put in Fredbear's mouth when the suit is in animatronic mode. The creations they were so proud of were sullied and tarnished--all of them stained with blood.