“‘You’ll stay with me?’
‘Until the very end,’ said James.
‘They won’t be able to see you?’ asked Harry
‘We are part of you,’ said Sirius. ‘Invisible to anyone else.’
Harry looked at his mother.
‘Stay close to me,’ he said quietly.
And he set off. The dementors’ chill did not overcome him; he passed through it with his companions, and they acted like Patronuses to him, and together they marched through the old trees that grew closely together, their branches tangled, their roots gnarled and twisted underfoot. Harry clutched the Cloak tightly around him in the darkness, traveling deeper and deeper into the forest, with no idea where exactly Voldemort was, but sure that he would find him. Beside him, making scarcely a sound, walked James, Sirius, Lupin, and Lily, and their presence was his courage, and the reason he was able to keep putting one foot in front of the other.”
J.K Rowling “Harry Potter and the Deathly Hallows”