as you can see from this moment in chapter 10 the twelve towers it is mentioned that sully used to be a squid, this moment could mean that he served in the us navy but we all know thats not true, its clear from his behavior in the flashback scenes in uncharted 3 that he is trying his best to hide his true squid form, as well as the fact that sully originally was meant to be a talking squid companion to nathan drake in the first game, so this line could not mean anything else

squid sully was born in the pacific ocean to a family of squids, the thing he liked to do the most was learn the language of humans by capturing a new unsuspecting child every day and forcing them to teach him english bit by bit, and a little of colombian since he lived underwater near to cartagena colombia, his family never approved of him learning the language of the humans since they are killing sea life and polluting the sea every day, but sully never listened to them

he has never been able to have a relationship as a squid since the squid population near south america is all getting captured and killed by china, and his strict parents would never allow him to marry someone or get a squid girlfriend, since his kind's religion has strange laws about relationships, his relationship with his family was never that good either, they constantly had fights and arguments and squid sully would run away from home several times, his brother squid gully would also pick on him a lot, until he unfortunately was killed by 2 colombian fishermen who in a moment of rage started shooting at the sea with machine guns screaming "I ♥♥♥♥♥♥♥ HATE THE SEA" and unfortunately several bullets hit the poor squid man, the family was never the same after that incident

after squid sully was all grown up he finally got a human costume for his birthday from his only friend, after this squid sullys life would never be the same, as he could walk around in the streets of cartagena colombia as a normal man and experience all the things humans get to experience, he started a romantic relationship with katherine marlowe and started his crime business, the one thing his parents would never want for him, but he was too far gone to listen to them, he stole, smuggled killed people, dealt with hector alcazar and met the street urchin nathan drake, who he mistook for ♥♥♥♥ ney and initially cowered in fear pleading for mercy (♥♥♥♥ ney was removed from the villains wiki for being too risky for the public to find out about his crimes, the ♥♥♥♥ ney scene was soon cut from uncharted 3 for the same reason), eventually he befriended nathan drake in the 2's search for sir francis drake's treasure, and although he has been hunted by the authorities everywhere he goes due to most of them finding out he is a squid cuz of the several people that have betrayed him in his journey, him and nathan developed a beautiful friendship

the events of uncharted 5 kingdom of squids, the unreleased uncharted game that was about sully's original squid life, after revealing the secret that he was a squid all along nathan is shocked but tags along with him in the underwater journey to find his old friend, squingo blingo, he introduces nathan to his family which don't approve of him having a human friend so he tells them about his new human life and they kick him out and call the squid authorities, which after finding out about are now hunting him down, after speaking to a few contacts him and drake discover that his old friend has also become a treasure hunter and is now looking for the secret city of atlantis for poseidon's spear, so they start looking for atlantis in an epic sprawling underwater adventure that tells a lot of unexplained lore about squid sully's life

squid sully in the end of uncharted kingdom of squids finds out that the human suit that squingo gave him was actually full of trackers all along and that is why sully and consequently drake could never stay out of trouble with authorities and other criminals hunting them down, squingo planned squid sully's downfall all along that with the help of nathan and his own intelligence never ended up happening, sully is heartbroken that his only friend in the squid world would betray him like this, they kill squingo and run away with poseidon's spear as atlantis starts crumbling like the end of every uncharted game, and sully decides to finally retire his human life and live peacefully SPOILERS in the beach near drake's house in the epilogue of uncharted 4, receiving visits from drake, elena and cassie every day