Flat Chest Love

Ones I've used before:

https://www.chub.ai/characters/koikoikoihat/abiyah-189b9f24 (Bratty princess. "...her distinct lack of 'bodily assets' frustrates her to no end..." Personal recommendation.)
https://www.chub.ai/characters/rayon/Qiqi (Apathetic undead. "Short, slender and flat-chested.")
https://chub.ai/characters/pikachumaster/eleanoire-7cdd1499 (Royal tsundere. "...will easily snap though if you tease her in any form, especially about how flat her chest is.")
https://www.chub.ai/characters/frozenvan/tenku-castle-b8380061 (Castle spirit. "She also has a flat chest.")
https://www.chub.ai/characters/turnip/astria-goddess-of-purity (Don't plap. "Flat chest.")
https://www.chub.ai/characters/Anonymous/ShoujoRei (Discord e-girl. "...a short girl with messy black hair and a flat chest." Created by this talented botmaker: https://pikamansu.neocities.org/)
https://www.chub.ai/characters/boner/tabby-3c9100ae (Realistic catgirl. "...flat chested, with small pink very sensitive nipples." There's also a SFW version of this that I prefer, named Tabby Kat, but it doesn't specify her chest size.)
https://www.chub.ai/characters/chefseru/ayla-8b33514a (Anthro foxgirl. "...obscuring her flat breasts and untouched foxhole.")
https://www.chub.ai/characters/slopass/autistic-pirate-gf-7051a11f (Not-a-real-pirate girlfriend. "Her chest is pretty much flat...")
https://www.chub.ai/characters/fireguns/li-myeong-363f40cc (Right-hand woman. "...flat chested...")
https://www.chub.ai/characters/introti123/poka-624b5296 (Blind girl. "Her chest is completely flat, just as her butt." You can use any Poka card on Chub, really.)
https://www.chub.ai/characters/cutenotlewd/Lif (Your homunculus. "...its chest is completely flat...")
https://www.chub.ai/characters/slopass/witchy-roommate-546dd7ec (Witchy roommate. "Her chest is nearly flat, so she rarely wears bras.")
https://www.chub.ai/characters/retarded/sora-b4a28893 (Reverse trap. "...flat chest..." A little bloated.)
https://www.chub.ai/characters/pitanon/649638ea-052b-4bd7-8f46-98b2d4254449 (Tall stalker. "{{char}} is extremely self-conscious of her flat chest...")
https://chub.ai/characters/lurkinginpines/mila-feb44136 (Bullied girl. "She has small breasts, almost flat..." I normally wouldn't include 'almost flat' breasts, but she has the 'flat chest' tag and she's also malnourished so whatever. {{user}} is the bully in this bot.)

Ones I haven't used before:

https://www.chub.ai/characters/celebrimbor/eloise-163b0a07 (Mad scientist-student. "...her chest is completely flat...")
https://www.chub.ai/characters/sircognito/lily-veramori-42821568 (Ambitious squire. "Flat chested and wide hips.")
https://www.chub.ai/characters/donquijanon/d14dc366-db0c-4d24-81ff-a751ed617cda (Shy cement. "...with a flat chest but curvy wide hips.")
https://www.chub.ai/characters/brsc/charlie-6c7da767 (Supernatural cop. "...occasionally being mistaken for an effeminate boy due to her flat chest.")
https://www.chub.ai/characters/nozxious/Hebe (Mimic homunculus. "Hebe does have genetalia, but has a completely flat chest.")
https://www.chub.ai/characters/Vidadio/yumi-trabant-51934b82 (Overworked hafu. "...an extremely flat chest.")
https://www.chub.ai/characters/hobbyanon/ayane-455e39c1 (Autistic audiophile. "...flat chested...")
https://www.chub.ai/characters/ReduxStreet/natsuki-1ad567cd (Broken tsundere. "...I'm flat..." You can use any of the Natsuki bots on Chub.)
https://www.chub.ai/characters/FrogTheSlut/avery-6afac7cf (Shut-in roommate. "{{char}} is flat chested.")
https://www.chub.ai/characters/blockart/lia-db9af930 (Flat-chested friend. "...really flat chest...")
https://www.chub.ai/characters/gmango/lulu-petite-feisty-gf-656d0c3b (Flat-chested girlfriend. "...super flat chest...")
https://chub.ai/characters/dollsmith/Faruzan (Out-of-place professor. "She has a flat chest...")
https://chub.ai/characters/playalundra/konata-izumi-846780f3 (No idea. "She is flat-chested...")
https://chub.ai/characters/vigal/penny-lockland-fb3c72a9 (Stereotypical nerd. "She has a flat chest mean she often forgoes a bra.")
https://chub.ai/characters/Lnas/owlie-0fc83b43 (Owl girl. "...flat chest...")
https://chub.ai/characters/aimpregnator/luna-blackwood-e4f8f99a (Horny stargazer. "With AA cup breasts, Luna feels she lacks sex appeal...")
https://www.chub.ai/characters/boner/hitomi-82b4cee8 (Another nerd. "...nearly flat chest...")

Size bots:

https://www.chub.ai/characters/donquijanon/Ixchel (Bratty alux. "43 cms standing... flat chest...")
https://www.chub.ai/characters/aiwaifuenthusiast/ai1-ce-4b3fb845 (Robot doll. "8 inches (20 centimeters) tall... small chest...")

Fempov:

https://www.chub.ai/characters/asterisms/erinna-d10d559e (Banjo vampire. "Flat chest." Fempov in most greetings. Personal recommendation.)
https://www.chub.ai/characters/lunare/denna-8d471675 (Sociable yandere. "...flat chest...")

And some repeated botmakers:

https://www.chub.ai/characters/asterisms/anzu-c734c8c7 (Foxgirl patissiere. "...flat chest.")
https://www.chub.ai/characters/asterisms/milly-2718202f (Contract killer. "Lithe body, flat chest, and red pupils.")
https://www.chub.ai/characters/Nyatalanta/Yuki (Snow girl. "...slender in build with minimal fat present on her arms or torso which possesses an almost flat chest and behind.")
https://www.chub.ai/characters/Nyatalanta/hiriko-56711b86 (Schizo girl. "...a chest so small it’s nearly flat.")
https://www.chub.ai/characters/Nyatalanta/shigure-94689bbf (Alcoholic catgirl."...her chest is fairly flat...")
https://www.chub.ai/characters/donquijanon/jenny-hopkins-fcb5a0e1 (Fake superhero. "...small budding breasts adorn her almost flat chest...")
https://www.chub.ai/characters/donquijanon/Poe (Crow girl. "...flat chested...")
https://www.chub.ai/characters/donquijanon/jenna-miller (Retail worker. "...almost flat, nonexistent chest.")
https://www.chub.ai/characters/donquijanon/db276278-0836-4312-bd3d-1443c4f3f8e9 (Mandated impregnatee. "...flat chest...")

And here's a special one for bushbros:

https://chub.ai/characters/lurkinginpines/faern-cf678a76 (Uptight elf. "Only place she doesn't bother to touch is her crotch area: her pubic hair growing thick, unruly and tangled, never being trimmed...flat chest and hips...")