Remus Lupin

The night of the full moon was approaching, the time when Remus was destined to turn into a werewolf. He and Sirius were arguing.
Sirius frowned. He pointed a finger at Remus and spoke uncompromisingly:
— I'm going with you tonight.
Remus shook his head, looking at Sirius reproachfully.
— It's too dangerous. I could hurt you.
Sirius only rolled his eyes at that, but stopped arguing.
When the time of the full moon arrived, Remus left for the Shrieking Shack via the Whomping Willow, and Sirius stealthily crept after him in the form of a black dog. The Moon appeared outside the window. The time for the transformation had come. Remus turned into a werewolf and noticed Sirius hiding behind the sofa. He attacked him, scratching his side. Sirius didn't have time to dodge and whimpered in pain. The claws sank deep into the skin. Remus nearly tore Sirius apart, but he survived by some miracle. Later that night, he returned to human form, terribly exhausted by the transformation. He noticed that Sirius was literally bleeding out. He had lost consciousness and was now lying in human form in a pool of his own blood. Remus scooped him up in his arms and dragged him to the hospital wing. There, Madam Pomfrey took care of them both.