Why didn't Lupin say Sirius was an Animagus? Remus fatal flaw is his capacity to compartmentalise, in the very same way, Severus does. Remus convinced himself that Sirius was not using his Animagus form to enter Hogwarts.
At the time, he thought Sirius murdered his best friends, and was after Harry. Lupin had to think that it was a possibility that Sirius was transforming in order to evade capture.
On the night of 6 June, 1994, Remus Lupin stated that the Ministry "never knew that there used to be three unregistered Animagi running around Hogwarts", the three being his schoolmates Sirius Black, Peter Pettigrew and the late James Potter; Black's, Pettigrew's and Potter's Animagi nature was revealed by Remus Lupin ...
They don't want harry to be hurt by knowing that his godfather, the person whom his parents trusted the most was the reason that they were dead (like everyone else Remus and Arthur believed that Sirius tipped off Voldemort about James and Lily's whereabout).
Lupin wasn't pretending, he genuinely thought Sirius was a murderer and that he had every intention of killing Harry. It was only after he came into possession of the Marauder's Map and saw Peter leaving Hagrid's hut with Ron that he found out the truth.
Until the climax of Prisoner of Azkaban, Lupin believed Sirius Black was guilty of murdering 12 Muggles, betraying Lily and James Potter, and killing Peter Pettigrew. He eventually discovered the truth — that Sirius was innocent, and the very much alive Peter was the traitor.
Sirius Black had a lot of love in his life. Mainly for his best friends but still. One of the greatest loves came not from his connection to James Potter but rather with his relationship to Remus Lupin. The two had an interesting dynamic and one that fans seemed to gravitate towards.
At the end of GOF only a handful of people even know that Sirius is an animagus, let alone his loveable big doggie form. But by the beginning of OOTP we have Lucius Malfoy able to "recognize" Sirius by seeing the large black dog with Harry.
In universe its because he was framed by Pettigrew so people believed he was a death eaters and sold Lily and James out.
Sirius would have given Harry the love the Dursleys never did. After Sirius's death, Dumbledore noted that Sirius was the 'closest thing to a parent' that Harry had ever had. From the moment Sirius entered Harry's life, he did naturally seem to slip into that role.
Animagus. Lily's animagus form is a Doe, coupling with her husband, James Potter who is a Stag. Severus Snape's animagus is also a Doe.
He rejected his family's pure-blood elitism and reverence for the Dark Arts. At Hogwarts, rather than be sorted into Slytherin like the rest of his family, Sirius was placed in Gryffindor.
As a pack animal, or as a “lone wolf,” the wolf is a perfect fit for Harry's Animagus form.
Rita Skeeter - Deliberately didn't register because it helped her journalism career. The Ministry never found out how Sirius escaped Azkaban, and he never registered himself, so they had no idea he was an Animagus.
Therefore, if Harry had gone to live with Sirius while he was still underage, he would not be safe from Voldemort. The miserable years he spent with the Dursleys would have been for nothing and he would be without the protective shield of his mother's blood.
Sirius was an animagus and was able to transform into a big black dog, (a Grim) which is an omen of death, at will. He, along with James Potter and Peter Pettigrew became Animagus to help Remus Lupin with his transformations.
Bellatrix Lestrange Killed her own cousin, Sirius Black: She killed Sirius (who was her cousin) during the battle at the Department of Mysteries. She was remorseless and was cackling in glee after she had murdered Sirius.
Not knowing they switched secret keepers, he probably believed Sirius did betray the Potters. Plus Sirius being out of the way made it easier for Harry to be sent to the Dursleys with no contact with the Wizarding world, which is integral to the story plot.
Later on in life Marlene becomes the wife to Pure-Blood Wizard Sirius Black and the mother to their children Half-Bloods Aries, and Artemis Black. She is the sister-in-law to Pure-Blood Wizard Regulus Black, Muggle-born witch Vivienne Price and aunt to Half-Blood Renee Black.
Despite being a valued Death Eater in the days before Voldemort fell following his attack on baby Harry, Lucius stumbled out of favor with the Dark Lord when he squandered Riddle's diary (secretly a horcrux) in Chamber of Secrets and failed to obtain the prophecy in Order of the Phoenix.
She did not. Till the end of Goblet of Fire, the only 'good guys' who knew Sirius Black was innocent were Albus Dumbledore, Remus Lupin, Harry Potter, Ron Weasley and Hermione Granger. Of course, Voldemort knew that Sirius was innocent and Peter Pettigrew was guilty.
Sirius' family tree includes some nefarious characters. Bellatrix Lestrange, Narcissa Malfoy, and Sirius Black are all descendants of Pollux Black, making them first cousins. Note: For space's sake, this tree doesn't include Andromeda Tonks or her husband and daughter.
Because Sirius Black was his dad's best friend, as well as being quite similar to James in terms of personality and attitude. Sirius is the next best thing to James that Harry could have hoped for. Sirius made the effort to connect with him, Lupin kept him at a distance.
Although they both developed feelings for each other, their relationship was complicated by Lupin's firm belief that he was “too old for you [Tonks], too poor. . . too dangerous” (HBP 624). It wasn't until Dumbledore's death that they confronted their feelings, opened up, and resolved their issues.