All of these characters have already appeared in the story. Most of them are already in the harem, and the rest are the ones I think will join later. I'll update this list once a month, if I forget anyone, just tell me in the comments.
—High School DxD
Rias Gremory
Akeno Himejima
Koneko Toujou
Sona Sitri
Serafall Leviathan
Raynare
Kalawarna
Mittelt
Asia Argento
Katerea Leviathan
Yubelluna
Grayfia Lucifuge
—The Testament of Sister New Devil (Shinmai Maou no Testament)
Mio Naruse
Maria Naruse
Chisato Hasegawa
Yuki Nonaka
—Mieruko-chan
Miko Yotsuya
Hana Yurikawa
—Redo of Healer
Flare / Freya
Norn
Anna
Lapiz
Guren
Kureha Clyret
Setsuna
Eve Reese
—Alya Sometimes Hides Her Feelings in Russian
Alysa Mikhailovna Kujou
Yuki Suou
Yumi Suou
—DanMachi
Haruhime Sanjouno
Alfia
Meteria
Astraea
Kaguya Gojouno
Alice Lovell
Hephaestus
Eina Tulle
Freya
Ais Wallenstein
Liliruca Arde
Ryu Lion
Adi Varma
Shakti Varma
Anya Fromel
Lyra
Dis sisters
Riveria Ljos Alf
Syr Flova
Airmid Teasanare
Nina Tulle
Hestia
—The 100 Girlfriends Who Really, Really, Really, Really, REALLY Love You
Karane Inda
Hakari Hanazono
—My Teen Romantic Comedy SNAFU (Oregairu)
Shizuka Hiratsuka
Yukino Yukinoshita
Haruno Yukinoshita
Yui Yuigahama
Mrs. Yukinoshita
Mrs. Yuigahama
Yumiko Miura
—Monogatari Series
Hitagi Senjougahara
—Jujutsu Kaisen
Yuki Tsukumo
Tsumiki Fushiguro
—The Demon Girl Next Door (Machikado Mazoku)
Momo Chiyoda
Sakura Chiyoda
Yuko Yoshida
Anri Sata
—Oni Chichi
Kayoko Akitsuki
Marina Akitsuki
Airi Akitsuki
—Tokyo Ghoul
Rize Kamishiro
—Dandadan
Momo Ayase
Slit-Mouthed Woman
—In/Spectre
Rikka Sakuragawa
Kotoko Iwanaga
Karin Nanase
Saki Yumihara
Hatsumi Nanase