Bleda appears as a Mangudai with a unique portrait and, prior to the release of The African Kingdoms, is not classified as a hero (Bleda is categorized as a common unit in the Scenario Editor). As a result, he did not possess the ability to regenerate health nor a hero's innate immunity to conversion until he was updated in the third expansion. Bleda can be garrisoned inside Rams.
Additionally, while Bleda uses the model of a Mangudai, he is actually a melee unit and benefits from normal cavalry upgrades like Forging and Chain Barding Armor; he has no range and cannot engage enemies from afar. Despite this, he uses the regular Mangudai attack animation, i.e. fires arrows at the target.
While the real Attila the Hun did rise to become the leader of the Hunnic Empire following his brother's death, the circumstances of his ascension were pretty much left ambiguous by historical records. Some sources states that Bleda was deliberately murdered by his brother in 445 after the Huns pulled their forces out of Byzantium, while others speculated it was the result of a hunting trip where an assassination attempt was made on Attila's life (and failed).
The hunting trip version of the theory was depicted in the first level of Attila's eponymous campaign.