All the characters Okuu could be thinking of, and accepted nicknames.
Overview
tl;dr: Characters in all windows games from EoSD through to UM and TGI, and some non-game characters. You don’t need to know any PC-98 characters.
Sections are listed in the order they appear in this game’s answer list, which is somewhat – but not quite – chronological order.
Main Characters
– reimu
– marisa
– marisa
Touhou 6: EoSD
– rumia
– cirno
– meiling
– patchy/patchouli
– sakuya
– remi/remilia
– flan/flandre
– cirno
– meiling
– patchy/patchouli
– sakuya
– remi/remilia
– flan/flandre
Touhou 8: IN
– wriggle
– mystia
– keine
– tewi
– reisen/udongein/udonge
– eirin
– kaguya
– mokou
– mystia
– keine
– tewi
– reisen/udongein/udonge
– eirin
– kaguya
– mokou
Touhou 7: PCB
– letty
– chen
– alice
– lily
– lyrica
– merlin
– lunasa
– youmu
– yuyuko
– ran
– yukari
– chen
– alice
– lily
– lyrica
– merlin
– lunasa
– youmu
– yuyuko
– ran
– yukari
Touhou 10: MoF
– minoriko
– shizuha
– hina
– nitori
– momiji
– aya
– sanae
– kanako
– suwako
– shizuha
– hina
– nitori
– momiji
– aya
– sanae
– kanako
– suwako
Touhou 7.5: IaMP
– suika
Touhou 9: PoFV
– eiki
– komachi
– medicine
– yuuka
– komachi
– medicine
– yuuka
Touhou 11: SA
– yamame
– kisume
– parsee
– yuugi
– satori
– rin/orin
– utsuho/okuu
– koishi
– kisume
– parsee
– yuugi
– satori
– rin/orin
– utsuho/okuu
– koishi
Touhou 12: UFO
– nazrin
– kogasa
– ichirin
– murasa/minamitsu
– shou
– hijiri/byakuren
– nue
– kogasa
– ichirin
– murasa/minamitsu
– shou
– hijiri/byakuren
– nue
Touhou 13: TD
– kyouko
– yoshika
– seiga
– futo
– tojiko
– miko
– mamizou
– yoshika
– seiga
– futo
– tojiko
– miko
– mamizou
Touhou 12.x: DS, GFW
– hatate
– sunny/sunny milk
– star/star sapphire
– luna/luna child
– sunny/sunny milk
– star/star sapphire
– luna/luna child
Touhou 14: DDC
– wakasagihime
– sekibanki
– kagerou
– yatsuuhashi
– benben
– seija
– shimmy
– raiko
– sekibanki
– kagerou
– yatsuuhashi
– benben
– seija
– shimmy
– raiko
Touhou 15: LoLK
– seiran
– ringo
– doremy
– sagume
– clown
– junko
– hecatia
– ringo
– doremy
– sagume
– clown
– junko
– hecatia
Touhou 16: HSiFS
– eternity larva/eternity
– nemuno
– aunn
– narumi/naruko
– satono
– mai
– okina
– nemuno
– aunn
– narumi/naruko
– satono
– mai
– okina
Touhou 14.5: ULiL
– sumireko
Various print works, group 1
– rinnosuke
– akyuu
– kosuzu
– akyuu
– kosuzu
Touhou 10.5: SWR
– tenshi
– iku
– iku
Touhou 14.5: AoCF; also Kokoro
– kokoro
– joon
– shion
– joon
– shion
Touhou 17: WBaWC
– eika
– urumi
– kutaka
– yachie
– mayumi
– keiki
– saki
– urumi
– kutaka
– yachie
– mayumi
– keiki
– saki
Touhou 18: UM
– mike/michael
– takane
– sannyo
– misumaru
– tsukasa
– megumu
– chimata
– momoyo
– takane
– sannyo
– misumaru
– tsukasa
– megumu
– chimata
– momoyo
Various print works, group 2
– renko
– mary/maribel/merrybel
– kasen
– miyoi
– mary/maribel/merrybel
– kasen
– miyoi
Touhou 17.5: TGI
– toutetsu/yuuma
Thanks to Goblin’s Globe for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.