Izuna is most likely one of the most mysterious character in the Narutoverse. He's truly a ghost. Barely seen but so important to Madara's storyline and his eyes the most powerful sight of the entire series. Regardless, all we know about his personality in the canon can be resumed in few sentences : He admired his brother as a child, he was also a fierce warrior from early age, he warned his brother against the Senju, and before his death willingly offered his eyes to Madara in order to protect the Uchihas. It's quite thin for a character development...
In the fandom, fanart, and the many fanfictions included, the general consensus is to represent Izuna as a soft and gentle character always devoted to his brother. Sometimes he's the victim of a hungry-power Madara (but that's really old stories back when no one knew that Obito was cosplaying Madara), often sassy and effeminate. A bit like Haku who was entirely in service of Zabuza. I suppose this idea comes from the adjectives that describes Izuna in the databook :
Considering the lack of information in canon, that headcanon starts to become the more popular. My ambitious is to try to discover what was Kishimoto's intentions for Izuna if he had more time to articulate that character. It's just an hypothesis that I present you, it's in no way a definite answer.
I) Intuition
That's nothing I can prove but it always seems to me strange that Izuna was the quiet one in this sibling's dynamic. If you put yourself in his shoes opened-toes sandals, he is Madara Uchiha's little brother. From birth, his brother was seen with special attention. Madara was the oldest, the heir and on top of that the genius one. Butsuma said that as a child he was able to kill adults. Being the second is virtually being invisible in front of other adults, it would have naturally created jealousy and a need to exist outside of the big brother's shadow. We've seen this same dynamic before between Sasuke and Itachi. The little one feels always left out. The difference there, is that Itachi was mostly absent in Sasuke's life, he was always busy with school and missions. And after the Uchiha's downfall, when Sasuke was just 7 years old, the distance between them wider dreadfully. For the majority of Sasuke's life, Itachi was a distant childhood memory. In contrary, Izuna seems to have stayed close to his brother until his death at 24 years old. They enjoy a childhood together and were so close than even adults they were able to take over the Uchiha clan. Because they spend more than 20 years together, sharing happiness and losses, there is for me an implicit idea that Izuna was balancing Madara and vice versa. In the same way, Hashirama's visionnary ideas are counter-balanced by a down-to-earth Tobirama. The Senju carves his unique personality by constantly creating new jutsus that surprised his opponents while Hashirama was known for being that rare and powerful Mokuton user. Madara being this taciturn person in public, Izuna must have been a bit more outgoing. So if Madara was in private described as kind by Hashirama, maybe Izuna was more rational when dealing with people.
II) Canon + extra materials
in extra materials I mean : databook, official card, official illustrations, video games ect...
Even if I'm team canon-only, sometimes other fillers or goodies can give us a hint of what would have been Izuna. And two things catch my attention :
first, he's always represented as someone very confident, holding a sword ready to slay someone, focused. Actually...he NEVER SMILES. He smirks once in a filler when he thought he was about to overpower Tobirama. It shows more his merciless attitude towards his enemy than any trace of softness. We are very far from the flower boy from fanarts.
secondly his voice...Have you heard his adult voice? It's surprisingly deep comparing to his slender appearance. Again it doesn't sound like a fragile baby doll but more like a commander in chief.
III) Izuna vs Obito
It took me a long time to realise how much Izuna is in the canon constantly in parallel with Obito. It goes back to my old post on "why Madara chose Obito". Months later, I think the answer is more obvious that what I thought back then.
Izuna simply might have had a very similar personality than Obito. Or should I say, Madara saw in Obito, not only his potential strength but also a clear resemblance in character to his dear brother. At the Kannabi's bridge, He observed and found Obito dying after he saved Kakashi, Rin and gave his right eye to Kakashi. There is a strong parallel to Izuna's life who also gave his eyes willingly to Madara when he was dying. In both case, there is a deep devotion for the well being of their friends/clansmen.
Even if Madara consciously chose to cut bonds with the world, unconsciously he still yearned for companionship. It's obvious by the fact he couldn't stop talking about his friend Hashirama, and in the similar way he looked for his "right hand" in Obito. Yes, in a deranged and manipulative way but even during the war, he never doubted Obito's intentions until it was obvious that he betrayed him.
So not only Obito was a replacement for Izuna as a brother in arm, but the databook gives them the same birthday the 10th of february, obviously the same zodiac sign Aquarius and most importantly the same blood type.
IV) Blood type
Izuna and Obito are said to be the same blood type O. Madara also is type O. As a westerner I've never paid attention to this information (I don't even know my own blood type to be honest!😅) until I've learned recently that Japaneses have got a whole culture associating blood type with certain personality traits. They take this more seriously that astrology to the point there is even campaigns to prevent discrimination towards rare types. Since I don't know much about this theory, let me quote some other sources.
I don't really know how to interpret the type O because the adjectives seems very larges and vagues to me. but it seems that leadership trait, hard working, outgoing, protective, direct, confident are the trait that comes the most from the various sources I've read. In Naruto, the others type O are : Kakashi, Asuma, Sakura, Temari, Raikage, Neji.
An other interesting aspect, is relationship between each types. Look closely at OxO relationship.
Let's go back now to compare now Izuna and Obito's databook pages with this new information in mind.
Notice again how Izuna is systematically showed in action, fighting someone or ready to attack. That's not really a moment where he looks relaxed with an easy-going attitude.
Izuna's favourite hobby was training with his brother Madara. Because of their closeness as brothers they developed an healthy competition. A good OxO relationship.
In the opposite, considering the circumstance of their meeting, Obito never truly trusted Madara and even when he agreed on the Tsukuyomi plan, he decided early on that he'll do it his own way and almost overtook Madara's role as the "saviour". It shows a negative OxO relationship.
In this perceptive, it seems to me that Izuna and Obito share in common of being extremely strong-willed and competitive. They have an inflexible determination that never sways. After being deadly wounded, Izuna still found the strength to warn Madara about the Senjus and again in his deadbed he made Madara promise to watch over the Uchiha clan. His vision (even in the literal way) was strong enough to motivate Madara long after his death. This devoted, balanced personality must be read as a confident desire to protect his people, the Uchiha clan, to the point of self-sacrifice.
Obito demonstrated two times an incredible will. Once as a child when he defended Kakashi's father and swore to destroy an unfair shinobi system, and a second time when he battled against the juubi. He knew he wasn't physically strong enough neither to hold the rinnegan nor contain juubi inside him but by cheer willpower he forced his destiny.
To conclude my guess is that Kishimoto intended Izuna as a warlord, a man of action, just like Madara, with a strong sense of duty and sacrifice for his clan. And the Senju represented to him the epitome of threat for the Uchiha's peace. Izuna pictured peace and harmony related to his clan not the whole world like his brother did (and maybe that's why he had to make him promise to protect the Uchihas first!?). He was probably more extravert, agreeable and also more distant emotionally than Madara. His interaction with his big brother must have been a funniest version to what we've seen with Obito. Just imagining those two uchiha big egos together, they were probably insufferable for their opponents 😂. If you read the manga again it's as if Madara is expecting a back-and-forth banter with Obito...just like Izuna would have done with him in the middle of a battles.😢
That's all I wanted to share on my hypothesis, maybe in future Kishimoto will give us something about Izuna's past and this post will age badly but let me know what you think of this theory?
"You were always at my side, the least I can do is to be beside you at the end. I know it cannot be, but I wish I could go to where you have gone. How I wish I could join you there, Haku."