In AC Origins, we meet Reda, and over time through his interactions with Bayek of Siwa, we get conflicting stories of his parents, which cast his true past in doubt...
Though he does not appear in Odyssey, he later shows up in Valhalla, looking more like a young teenager rather than a boy. This resulted in lots of speculation in the Valhalla Discussion Forum
regarding his feathers or his orange scarf/sash possibly being some form of Isu tech to slow his aging over the course of roughly 900 years... Regardless, he had clearly developed his industry from a one-man operation in Ptolemaic Egypt to a far-reaching ‘shadow market’ with numerous helpers...
There was much speculation that Ubisoft would resolve the mystery of Reda in the Valhalla storyline through a content update. However, this issue became even more convoluted when the Dawn of Ragnarök expansion was released, and Reda’s presence in Svartalfheim completely changed the context of his existence...
All the sudden, Reda went from being some unassuming opportunistic immortal to likely being an Isu that survived the Great Cataclysm in a fashion unlike the other methods used by the Isu to reincarnate themselves. With the release of ‘The Last Chapter’ content in Valhalla, no new insights have been addressed about Reda. The story of Mirage takes places decades before Valhalla, so it is likely Reda will show up in Baghdad, but there will need to be some piece of story to conclude the mystery of Reda in that period.
Perhaps in Baghdad we will see him start up the Thousand Eyes, which would go on to influence or become a part of the future Assassin Brotherhood after the Hidden Ones dissolve? Or, perhaps there will be a present-day resolution that reveals Reda had been staying underground for some 1000 years (which would explain his absence in other AC games) and has him play a pivotal role in the grand story arc started by Layla in AC Origins? The speculation continues, but whatever the case, I sincerely hope the mystery of Reda is concluded in Mirage.