Rollo was "hostile", but Rachel was able to earn his trust over time by giving him scraps of food she saved from her lunch. He grew increasingly agitated and unhappy kept chained up for most of the day, until Rachel couldn't play with him without him hurting her.
An abandoned puppy, Rachel rescued Rollo and kept him a secret from her abusive stepmother, who found the puppy and attempted to drown him. Rachel triggered and empowered Rollo. Unable to control him, he killed her foster mother, maimed her foster siblings and destroyed the house.
It came to me when I was trying to puzzle out why Bitch's power works better on Bastard [...] shards imprint on that which is closest, taking information from the surroundings at the time of trigger. Wildbow never actually describes what Rollo looks like. I checked. Rollo is only described as wanting a lot of freedom and becoming increasingly aggressive (even towards Rachel) as time goes on. While that's certainly true of a cooped up dog, that would be more true of a wild dog. What if Rollo was actually an abandoned wolf pup? [...] Her power imprinted on a wolf, works best on wolves. Still works for wolf descendants, but not as well.