(warning spoilers ahead!)
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Cloud was infused with Mako Energy during his and Zack's Nibelheim capture. So they were experimented there for 4 whole years. Zack's body was too strong, so they couldn't complete the experiment using his body because his own natural strength kept rejecting the Jenova Cells and Mako, but still, he was weakened because of the experiment (dba nung nilalabanan niya yung mga soldiers sabi niya nanghihina na siya?)
So, they used Cloud instead. Cloud's body was perfect for them for experimentation because he was weak mentally. So Cloud became the perfect guinea pig, sobrang pinag-experimentuhan siya, pinuno siya ng mga Jenova cells at Mako.
Finally, after 4 years, Cloud and Zack escaped, with Zack carrying Cloud all the way back to Midgar kasi gulay-gulay si Cloud (na Mako-poisoning siya). Zack wanted to meet Aerith/Aeris once again (yung letter sa ending ng Crisis Core). The SADDEST part was that they were almost there! Zack and Cloud was almost home, but troops stationed just outside of Midgar ambushed them. This is where the last battle of Zack took place, fighting legions of soldiers, pero dahil nga dun sa experiment, nanghina na siya, so hindi niya nakayanan lahat. When the final blow was given to Zack, dun NAGISING si Cloud, just in time to hear Zack's last words.
"You'll be my living legacy." <--- ito yung sinabi ni Zack kay Cloud. Because Cloud was suffering from mental disorder, iba yung pagka-interpret niya (mejo baliw-baliw siya nung time na yun dba?). What Cloud did was, HE MIXED UP HIS AND ZACK's FEELING TOGETHER! So, during that point on, nung nawala na yung pagka-gulay niya, dahil yun sa nahalo na yung memories nila ni Zack.
Nakita ni Cloud na suot niya yung SOLDIER uniform (just like Zack) and hawak niya yung BUSTER SWORD (Zack's Sword). So Cloud thought that he joined SOLDIER (pero sa totoo hindi), at dahil sa nahalo yung memories nila ni Zack, he thought he was in 1st Class.
A week after Zack's death, Cloud arrived at Midgar, where he collapsed at the train station. Nakita siya ni Tifa dito. Si Tifa naman, nung nasunog yung Nibelheim, dinala siya ng Martial Arts teacher niya papuntang Midgar. Tifa was part of AVALANCHE, yung group ni Barret na anti-Shinra. Knowing Cloud's potential, she insisted Barret to let Cloud join. Some time after, Cloud's first mission as an AVALANCHE member began (ito yung nasa taas siya ng train).
hanggang disc 2 ng FF7, Cloud still had Zack's memories inside of him, kaya rin siya nagustuhan ni Aerith, kasi she saw a lot of Zack within Cloud. It was Tifa who reminded Cloud who he really was nung gitna ng Disc 2. So from mid-Disc2 and onwards, it was the pure Cloud who has with Tifa, not the Cloud mixed with Zack memories.
the reason why Sephiroth was able to control Cloud during nung game was because of the jenova cells within Cloud. tapos, Cloud really kept thinking na 1st class SOLDIER siya, because he has the uniform, the sword, plus nung inexperimentahan siya, nilagyan siya ng Mako energy, and every SOLDIER members are infused with Mako, so this really convinced him that he was part of SOLDIER.
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erm, that's about it. if di niyo nagets, hanap nalang kayo ng mas magandang summary sa net. ahaha! hope this helps guys.