And you know that there's a place in the sun

in the downpour of the battlefield, a requiem resounds

Becoming a SOLDIER took a lot-- a lot of strength both physically and mentally to withstand the exposure to Mako in its raw form, as well as injections of Jenova's cells. The result are soldiers with super human strength and abilities, however, with great strength comes great responsibilities. And Angeal serves as a teacher, imparting values, ethics, morals, and all that warm fuzzy wholesome goodness to his fellow SOLDIER: Class 1^st.

that strong, kind profile that I looked up to as a kid

Zack left for Midgar at the age of 13-- barely a teenager and would have needed a role model and someone to take a fatherly role as his parents were far away in Gongaga. Knowing Angeal helps us to understand why Zack did what he did. During the mission in Nibelheim, Zack no longer knows what is going on, Angeal isn't there to tell him what's going on, and he begins to feel like You know what? I'm a SOLDIER, I don't need to think or question what I'm doing, I only need to follow the orders given to me, and Shinra being the corrupt corporation it is, Zack would have been going down the wrong path. But remembering Angeal's words reminds Zack that his pride is something he carries with him, not something that Shinra can give him or take from him, so he must do what he knows in his heart is right. He could have turned tail and reported to Shinra when Sephiroth went berserk, but he stays and fights. Once he broke free, he could have ran off and left Cloud in that tank to be experimented on for God knows how long, but he doesn't. He could have realized at some point that Cloud was dead weight, and left him behind saving himself, but he doesn't. He sacrifices his life to try to get them both to to Midgar safely, he refuses to accept that both of them can't make it there together. Why does he do these things: for Angeal and because of the values Angeal instilled in him.

the fragile hero, can be anyone if there is something that must be protected

When Zack was in the holding tank in the basement of Shinra Mansion and has a vision of Angeal walking away from him, disappointed at how weak Zack had become, Zack is desperate. He bangs on the tank, calling out to Angeal to get him to come back to not turn his back on him. He cares a great deal of Angeal's opinion of him. That's an understatement. Whether Zack seeing Angeal was an illusion brought forth by his own mind or if Angeal had anything to do with it is hard to say. It could have been Zack's subconscious not wanting to fail Angeal or tarnish the way Angeal sees him. Or it could have very well been Angeal, communicating through the Lifestream, and giving Zack enough tough love to motivate him to fight back and not give up.