Peter being jealous of Barty Jr. during GOF because he’s the dark lord’s favorite, but also being attracted to him because he’s powerful, and smart, and oh so good at serving their master.
Barty Jr. is visiting his father’s house, and he’s grading papers while Peter writes to Percy at the ministry pretending to be Barty Sr. They get bored, and start up a conversation with each other. At first they talk about the plan to resurrect LV, but then conversation shifts to what they plan to do when he wins the war, what a future could look like. They start flirting, and one things leads to another and they fuck in Barty Sr.’s bed.
They continue their liaison. LV finds it amusing because who would want to fuck Wormtail? He recommends that Barty Jr. drop it, but Barty Jr., for once doesn’t listen. LV isn’t angry with him, but he insults Peter about it when Barty’s not around.
They stay together until Barty Jr. gets his soul sucked out. Snape also insults Peter about this, and he taunts him about while he makes Peter do chores for him during the summer of 1996.