I know my current (Pathfinder, Aberrant/Exalted) GM is doing his best to take the record, but I remember playing weekend-long sessions when I was a student, so the longest session would be part of Kella's story.
As mentioned on Day 12, she's one of my favourite characters of all time and a large part of that was down to how immersed we as players became in the game, which was assisted by being able to play for extended periods: I shared a house with the GM (Sith) and his girlfriend (Penny), who played Kyann, and our friend Owen (Rax) was the other main player and practically lived there too - and when Rowan (Reisha) joined he was just around the corner too. I know we needed to cram a lot in to make sure we finished the game before I finished uni, so I suspect the final session was the longest.
I've never played Final Fantasy X, the inspiration for this game, so I don't exactly know how it deviates. Our game was set 1,000 years or more before the computer game. The world has a strange mix of magic and and technology-esque items - leaning more towards magic than the world I remember from my foray into Final Fantasy VII, although that is a different place. Kella was Besaid born and bred, golden-brown skin, long hair thick and dark, and a love of Blitzball - the sport that unites the world and is particularly popular in her corner of it. She has a stocky, athletic build and plays passionately, but it's her younger sister Elin who has a real talent for the game. By the end of the game, she'd progressed from white mage to summoner.
Rax (short for Sycorax) was a chocobo breeder, as I recall. He became convinced he'd died and wanted Kella to send him on - I remember the GM saying he was a bit worried because Rax hadn't died but he was so convinced of it, the GM half expected if I tried a sending it would work!
Reisha was a man out of time. When the party was attempting to convince Phoenix to join them as part of Kella's available summons, they found him or Phoenix returned him to the world and left him to their care - I was never entirely sure which, or if something else entirely was going on. I think this was the first time he muttered "I could get used to waking up next to you" on seeing Kella - and then blushing bright red on realising he'd spoken out loud.
Whilst Reisha was the oldest player character in terms of time since birthed, Kyann was oldest in time lived and in maturity. She was a strong, steely woman - good in a fight, but her strength was predominantly that of will. A bit of a loner, she accepted the company thrust on her by fate and circumstance before realising she was fond of the others - close enough to call them friends.
It was her strength of character that meant Kella could survive her final encounter.
It's sadly hazy now, but I do remember avoiding an intended boss battle with the Behemoth King. I'd picked up a ring that enabled me to talk to animals and we ended up recruiting him as part of the party. We climbed Mount Gagazet where Reisha and Kella finally admitted to their mutual attraction. Kella had just enough time to realise she was pregnant before Reisha was killed (?) to save the party - by this time, everyone knew they had to get Kella to the top of the mountain, had to keep her alive so she could sacrifice herself...
It was revealed to them - a vision from the future (characters in the computer game) - that Kella didn't need to die. She chose Kyann to be her final summoned creature - the one destined to kill her. Kyann was taken over and indeed went to fight - but she was fighting within herself because she was determined not to kill her friend - and she won, staying the sin growing within her.
The Behemoth King returned to his herd and the surviving party - Kella, Rax and an NPC, Natasha (daughter of another NPC who'd died along the way) - returned to Besaid for the epilogue: Rax, who'd accepted his life during the final battles and who'd fallen in love with Elin before that, was married to Kella's younger sister and they had twins - Rain and Sky. Rax was on the beach with Kella, watching Elin play Blitzball and watching his twins and Kella's son, Zet, when something fell from the sky, and Kella knew it was Reisha coming home to her - and of course it was. And that's what the picture I chose for Day 12 illustrates. I wrote the scene as a fan-fiction after, and it still makes me cry to read it.