The Data books say one of Izuna's character traits is that he is "harmonic" but what does that even mean? was he like, super chill? Is he a member of the uchiha barbershop quartet?
He really was given the fridged female character treatment. Bet if we let Madara have some flashbacks of him, it's gonna be Izuna smiling in the middle of a field under the blue sky or some lame shit like that.
I've decided to take it in the sense that he, naturally and unconsciously, matches the freak of whoever's closest. Amazing improv skills. He's got the ability to become whatever Madara needs of him. Madara remembers a devoted, serious, loving little brother. He looks back and sees a small Izuna following him around like a duckling, a teen Izuna getting into mischief (when Madara needed some relief after abruptly becoming clan head). An adult Izuna, always looking ahead, staunch in his beliefs, never faltering even when Madara did.
Tobirama remembers a fucking beast. An all around deranged dude. Tobirama, Senju heir and sometimes necromancer, needed someone that made him look sane in comparison and Izuna delivered.
Hashirama, who never got close enough to Izuna for some strange chemical reaction to happen, observed this from afar, convinced Madara had two little siblings; Izuna and his twin, also named Izuna.