who were 'closer than friends closer than brothers'

"I wonder... what makes Albus Dumbledore so fond of you, Mr. Scamander?"

This quote hurts me so much. I think- no, I know- that this is one of the biggest reasons Grindelwald is so harsh when sentencing Newt. This is a boy who Albus Dumbledore (Gellert’s first love and arch enemy and original comrade in arms and the man whose sister he may have killed) had obviously taken a liking to.

God, he must be so conflicted. He and Albus, they used to be closer than brothers (certainly closer than Albus and Aberforth). Anyone Albus hated, he hated too. Anyone Albus loved, he loved too. So this Newt… this Newt. This Newt was mentored by his arch enemy, but also his ex best friend. This Newt gained Albus’ affections when Gellert himself had lost them.

And have you noticed Dumbledore’s affinity towards people who love animals? Everyone keeps bringing up Hagrid, but you must remember the first person in Albus’ live that spent almost all of his time with his goats, and while Albus and Aberforth were never close, you can’t tell me he didn’t love his brother. Gellert can see that. He saw what Albus liked in this kid, and it wasn’t just the shaggy blond hair and the revolutionary ideology that he’d taken a liking to, just as with Grindelwald.

It was the creatures. Gellert knew it was the creatures, taunted Newt with that, and reminds us that he knows Dumbledore, not just as an enemy but as a friend and possibly even as something more.

Gellert is jealous and vengeful and I believe, in this scene, he is motivated as much by hatred of Albus Dumbledore as he is by Newt Scamander.

The result of these shared experiences was a closeness unknown to all outsiders. Comrades are closer than friends, closer than brothers. Their relationships is different from that of lovers. Their trust in, and knowledge of, each other is total. They got to know each other’s life stories, what they did before they came into the Army, where and why they volunteered, what they liked to eat and drink, what their capabilities were. On a night march they could hear a cough and know who it was; on a night maneuver they would see someone sneaking through the woods and know who it was from his silhouette.
—  Band of Brothers by Stephen E. Ambrose