It’s many long minutes before Sirius begins to calm down and many more minutes of him apologizing for leaving Harry, ending up in Azkaban, not being there for him. Thankfully, he’s willing to accept Harry’s unconditional forgiveness with only some hesitation, some doubt over whether he deserves to be forgiven or not. Harry assures him that he is most certainly deserving of forgiveness and Harry has always been ready to offer it.