Okay! As you have read from the title description, this is a complete list of those had perished in the series. I recommend that you keep this one for future reference, for this is a preview for what could potentially happen in the future.
From time to time, I may decide to add on more characters onto this list, so don't hold your breath!
Casualties of Thanos
• Hayner
• Pence
• Xion
• Roxas
• Fuu (outside of story)
• Rai (outside of story)
• Dilan
• Aerith
• Cid
• Abu
• Prompto (outside of story)
• Gladiolus (outside of story)
• Ignis (outside of story)
• Hope Estheim (outside of story)
• Will Turner
• Elizabeth Swann
• The Princesses of Heart (excluding Kairi)
• Nala
• Li Shang
• Yao
• Ling
• King Triton
• Flounder
• Cogsworth
• Sally
• The Fairy Godmother
• The Three Good Fairies
• Megara
• Phil
• Mrs Potts
• Pumbaa
• Eri
• Beat
• Dale
• Huey
• Dewey
• Louie
• Clarabelle
• Pluto
• Darkwing Duck (mentioned)
• Max Goof (mentioned)
• Roxanne (mentioned)
• King Mickey
• Goofy (outside of story)
• Aqua
• Terra
• Ventus
• Master Yen Sid
• Wakka
• Riku
• Cloud
• Sora
Casualties of Sephiroth
• Some random bullies
Survivors
• Olette
• Naminé
• Lea
• Seifer (outside of story)
• Setzer (outside of story)
• Vivi (outside of story)
• Squall
• Yuffie
• Aladdin
• Braig
• Simba
• Ansem the Wise
• Cor Leonis
• Aranea
• Ienzo
• Aeleus
• Isa
• Mulan
• Chien Po
• Ariel
• Sebastian
• Timon
• Rafiki
• Lightning (outside of story)
• Noctis Lucis Caelum (outside of story)
• Captain Jack Sparrow
• Jack Skellington
• Neku
• Shiki
• Rhyme
• Warrior of Light
• Firion
• Luneth/Onion Knight
• Cecil
• Bartz
• Zidane
• Sanae Hanekoma
• Joshua
• Queen Minnie
• Enna Kros
• Shantotto
• Sarah
• Donald Duck (outside of story)
• Chip
• Daisy Duck
• Horace Horsecollar
• Clara Cluck
• Scrooge McDuck (outside of story)
• Gosalyn (outside of story)
• Vaan
• Tidus
• Tifa
• Selphie
• Kairi
• Rinoa
• Terra Branford
• Garnet/Dagger
• Serah Farron
• Yuna
• Lunafreya
• Vanille
• Y'shtola
• Rydia