Can't sleep so here's the whole of Ienzo's storyline written on mobile from memory.
Ienzo is not originally from Radiant Garden. He has vague abstract memories of hearts but no real strong memories. It's unknown if he lost his parents prior to coming to Radiant Garden or after. It's also unknown if Ienzo's parents are dead or just separated. He just "lost" his parents.
Ienzo was adopted by Ansem The Wise and raised by all the apprentices. Ansem TW and Even doted on him the most with both becoming his main guardians. It's also mentioned that after Apprentice Xehanort arrives Xehanort took Ienzo under his wing as a student.
Ienzo some point after BBS suggests to Ansem The Wise to build the labs and experiment on Xehanort. This leads to experiments on themselves which leads to the apprentices + Lea and Isa falling to darkness. Ansem TW is cast into the realm of darkness. Shortly before this but unclear when, Ansem TW was experimenting on children. It is unclear if Ienzo was one of them or not. Ienzo is still a child during all of this.
Ienzo's heart was cast off and became a heartless but his body remained. It became Zexion who grew up within the Organisation 13. Originally Zexion was second in command to Xemnas outranking Saix. Zexion was questioned by Xigbar over what Xemnas' secret was before being given the information of Ventus being located in Castle Oblivion. Zexion would be later sent to Castle Oblivion to work on the Replica programme and help find the traitors.
At Castle Oblivion Zexion helped create Xion and Replica Riku alongside Vexen and used his scent/serene gaze ability to track everyone in the Castle. It is unknown if he knew DiZ was Ansem the Wise and just omitted the information. Regardless Zexion reported and followed the whereabouts of multiple people. Following the death of his two allies, Zexion decided to confront Riku. He took on the form of Sora and fought with Riku and trapped him in light. Riku almost succumbed to the light which is suggested to not be an illusion before Namine saved Riku and convinced him to accept the darkness. After accepting darkness, Riku wounded Zexion and made his disguise as Sora fall. They fight further and Zexion retreats. Back in the basements of Castle Oblivion as Zexion recovers, Axel appears with Replica Riku and gets Replica Riku to absorb Zexion's power, killing him. It turns out Saix sent Axel as an assassin to kill Zexion so Saix could take his place and get more information on Subject X.
Some point after the events of Kingdom Hearts 2, Ienzo is recompleted. He is now late teens/young adult after his body having grown up as Zexion. He helps get everything back on track and begins taking over Ansem The Wise's role. Ienzo begins helping Sora free the hearts trapped inside of him but omits the information on Ventus' location. If he was aware Apprentice Xehanort was Terra's body like suggested in BBS, he also omits this information. He behind the scenes, with help from Demyx and Vexen, creates replicas for the hearts in Sora to return to.
After the events of KH3 he helps Kairi dive into her memories to find out where Sora is.