Before the events of Crisis Core: Final Fantasy VII, a women named Gillian Hewley worked with a Shinra scientist named Dr. Hollander in a project called Project G (or Project Gillian), attempting to create a Cetra-Human hybrid. Gillian was injected with Jenova cells (who Hollander believed to be a Cetra), and then her cells were injected into other subjects. The experiment was mostly a failure, with all of the subjects giving birth to unsuccessful hybrids, such as Genesis (whom would go on to become great friends with Angeal), however Gillian gave birth to Angeal, the experiments only success. Unlike the other subjects, Angeal does not degrade, although like the other subjects, he does not have Cetra-like powers. The most notable difference between Genesis Rhapsodos (a failed subject) and Angeal (the successful one) is that Angeal's is white, whereas Angeal's is black.
Many years after this experiment, Angeal Howley is one of the three 1st Class SOLDIER members in Crisis Core Final Fantasy VII before Zack Fair is promoted (along with Genesis and Sephiroth). Angeal acts as Zack's mentor and friend, teaching him about SOLDIER, training him and constantly reminding him to embrace his dreams and to never forget his honor. Angeal is the original wielder of the Buster Sword (he passes it down to Zack, who passes it down to Cloud Strife), although he rarely uses it because "use brings wear, tear and rust".
While Zack and Angeal are in a mission in Wu Tai, Angeal mysteriously goes missing. Everyone within SOLDIER immediately suspects that he is in league with Genesis, who recently disappeared and turned into a renegade member of SOLDIER. Zack denied this, claiming that Angeal would never do something like that. However, upon being promoted to SOLDIER 1st Class, Zack is sent to investigate Genesis' disappearance. When Zack finds him, he confronts Genesis, only for Angeal to appear. Angeal takes Zack's sword, and points it at Gensis, therefor declining Genesis' apparent offer of joining him. Genesis tells Angeal that he respects his wishes, but asks him if he can "really live in that world". Angeal plunges the sword into the ground and exits. Zack goes to Angeal's mothers house in order to find him, only to find Gillian (Angeal's mother) lifelessly laying on the ground, with Angeal behind her. Zack punches Angeal and demands and answer as to why he did this, but he is interrupted by Genesis, and Angeal exits once again.
Later in the game, Zack finds Angeal in Modeoheim, and Angeal fuses with four Angeal clones, becoming "Penance Angeal". Angeal is defeated by Zack, and a dying Angeal tells Zack that he is proud of him, and that Zack must do the rest (in their quest to rid the world of torment). Angeal's final words were "Protect your honor, always", as Zack inherits the Buster Sword from Angeal.
An Angeal clone later appears in the game, helping Aerith fight off an attacking Shinra robot. The same clone appears at the end of the game when it helps Lazard (in Angeal's body) fight off Shinra officers who were after an unconscious Cloud. Upon Zack's return from battling Genesis, he finds the Angeal clone laying on the ground, dying but it quickly dissipates into the Lifestream with Lazard. It is likely that this clone was a representation of Angeal's will, and Angeal was likely controlling him from the Lifestream. Angeal makes a final appearance in the ending cinematic of the game, where he descends from above to pull Zack into the Lifestream with him.