The thing about Sirius Black is, it would have been so easy for him to become bitter towards Harry and he never did.
He was bitter about so many things in his life, but his love for Harry never wavered.
And just think about it, James and Lily had died for Harry. And we all know Harry had a lot of faults, and he wasn’t an exact replica of his father, which is how Sirius sometimes wanted him to be.
Yet Sirius never took out his resentment, his unhappiness, his suffering on Harry. He was never bitter towards him. He jumped right back into Harry’s life after 12 years in depression prison just as ready and willing to be a parent-figure has he had been the night at Godric’s Hollow when he found his best friend’s body lying cold and motionless in the front hall.
And Sirius’ love for Harry is really quite amazing, when you think about it.