Peter's betrayal makes him the central personal enemy of Sirius.
Sirius and Peter are revealed as former associates connected to Harry's parents. Peter framed Sirius and later escapes after his betrayal is exposed.
Ron unknowingly shelters the disguised betrayer as his pet.
Ron believes Peter Pettigrew is his pet rat Scabbers until the group discovers that Scabbers is Pettigrew's Animagus form.
Pettigrew is directly responsible for the betrayal that shaped Harry's central conflict.
Peter confesses that he betrayed Harry's parents, framed Sirius, and faked his own death. He then escapes before Harry can bring him to justice.
Lupin is connected to Peter through their shared past and the revealed betrayal.
Lupin is present when Peter's identity and confession are revealed at the Shrieking Shack, and he participates in exposing Peter's betrayal.
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Related entry on Wikidata (Q1000118), public domain (CC0).