Interest Stacks

AKKO ED
Other ~ Shin Angel (1996 H-OVA), To Heart IN (Same as VN OP)

Haruhi Terada
Koihime (2000 H-OVA), One: Kagayaku Kisetsu e (2001 OVA)¹, Tokyo Underground ED&IN (2002; First TV appearance), Flutter of Birds: Tori-tachi no Habataki (2002 H-OVA)
Did 1 Fate/Kaleid image song since changing name to Haruhi Nanao in 2014/5/30~

¹ One OP singer Millio also pairs with Haruhi Terada on Koihime, singing EDs of the sequel Zoku Koihime

Natsumi Yanase ~ First solo anime ED
Other ~ Ojousama Sousamou ED (1996; as part of the unit Virgo), Lamune INs (Itsumo yori Aoi Umi & Hitomi Sunshine), ef: a tale of memories. ED3 & melodies final ep ED

Millio OP, Haruhi Terada ED

KOTOKO

YURIA ED & IN

NANA OPED in OVA / Hiromi Satou OP & Yuria ED in 2003 TV

Maiko Sakurai OP & UCO ED (same as VN)

Ayumi Murata
Other ~ Kappa no Kaikata ED (2005; first non-vn), Lamune ED1, Happiness OP, Kono Aozora ni Yakusoku o ED, Kenkou Zenrakei Suiei-bu Umishou ED, Macademi Wasshoi! ED

Dorechu
Other ~ Karin & REC (under BRACE;d), Twin Angel (under ave;new)

Haruka Shimotsuki ED (also has anime songs under kukui alias)

Ayane OP

Ui Miyazaki OP

Riryka (as Miruku Kurumi) IN (花が咲く景色の中で)
Other ~ Venus Versus Virus OP1 // Shounen Onmyouji ED1 lyrics (as Hiroki Yuka), Sakurasou OP2 lyrics

Yui Sakakibara ED

Rekka Katakiri ED

mao/Duca OP3 2007/Jul/04
Used as movie OPs starting in 2008 with Doraemon Movie 28: Nobita to Midori no Kyojin Den

Ayumi. ED2 (as Yozora Orihime with AiAi) ; also has a YURIA (as Honey Bee) ED1
Other ~ Rail Romanesque ED, 180Kimimimi ED

fripSide OP/ED (nao's only anime series with fripSide. Also in Terraformars Revenge ED3 & Neptunia as a solo artist)

kicco (also sings the VN OP)

Rita (as Riko Hirai ; IN Fairy Dang-Sing~月下に妖精は舞う duet with Reina Kaihara)

Notes on previous MAL-listed works:
* Papillon Rose (2004) isn't listed as she doesn't sing in the version that is on MAL.
Crediting is a bit inconsistent across sites since there's also an internet version released 2002 (with Rita), a doujinshi (R) & a movie (G with OPED by Yui Sakakibara). Rita (as Mahiru Sonoda) is credited for OP lyrics & sings the web OPED & cast IN.
https://vgmdb.net/album/62236 web OST (Hikaru Nakayama) / https://vgmdb.net/album/90352 OVA OST
Web OP/ED --> https://youtu.be/GJqoqca0SY0 / https://youtu.be/gRSvT4eT4Vw
Web IN --> https://youtu.be/Kh0X-A651-I (the intro instrumental plays ~15:20 in the OVA & is the original web ED?)
OVA OP/ED --> https://youtu.be/00_FhP1mCwo / https://youtu.be/AazbsUM_Z34

https://web.archive.org/web/20020403073043/http://www5b.biglobe.ne.jp:80/~novy/html/sound_frame.html

* Voices Hikari in Wind: A Breath of Heart (2004) but only sings in the VN version.

* H-anime adapt Harukoi Otome (2006) https://youtu.be/o8E-YHhY5Vg (as Rita (Blueberry&Yogurt); Same as VN OP) Probably the earliest Rita credited vocal on MAL

Ringo Aoba IN & ep12 cast ED
Other ~ Euphoria (2012), Araiya-san (2019; also plays as IN in Overflow)

Hana ED1 (Grisaia no Rakuen Prologue OP; the classic Hana/Szak vibe)
> https://youtu.be/5ALmSHDK6Tg Masami Okui VN OP (so she's mentioned somewhere in here;

Mami Yanagi ED1

Haruna Ooishima (also sings VN OP)
Other ~ Tenchi Muyo s4 OP3, Akkun to Kanojo OP1&2, Duel Masters King Max ED (as duo)

Crazy cast. (I think the first anime where Duca appears instead of mao)