MADARA AND IZUNA (HEADCANON)
I suppose a person who had known Madara from the Warring States period, from his birth up to his twenties, would have been SHOCKED to discover the older version of Madara from the Naruto era as the epitome of the lone (yet dangerous) wolf.
Madara in his youth was ALWAYS with his brother. They were known throughout all the shinobi world as just "The Brothers", without needing to specify which clan we meant. They were constantly moving together like a sword and its sheath; on the battlefield, on missions, or ruling over the clan. One started a sentence, the other could finish it. At some point they looked so much alike that people often mistook their names, thinking they were twins, and I wouldn't be surprised if Madara grew his hair extremely long as a way to look visually distinct from his brother — he loves Izuna, but he loves being glorified by HIS NAME even more!
Yes, Izuna was his most precious brother, but also the only person in that cruel world he could trust with his eyes closed. They trained together from the moment both of them could walk, and as a result, part of their fighting style was designed to be performed as a duo. Even if Madara was naturally more creative and aggressive in his presence, all his war strategies took into consideration having Izuna as his most loyal ally. This dynamic was so ingrained in Madara's muscle memory that he unconsciously tried to revive it with Obito when he protected him from an attack (Obito didn't need it at all with his mangekyou's ability), then offered an alliance to Sasuke just because his face reminded him so painfully of his dear brother (but he quickly closed his heart and mercilessly stabbed him later).
Both Obito and Sasuke rejected him. And even if they had accepted, it was a stillborn project — no one could ever replace Izuna. When he passed away, Madara had to reinvent himself... alone. Or maybe the real Madara had died long before, resting forever inside Izuna's coffin. What remained alive was only the Ghost of the Uchiha.