MonokiniXL

v2 makes significant improvements to back and side angle views, breast size specification, and pose variety.

This LoRA produces a single style of monokini (sometimes called a "slingshot bikini," although I don't know why because by definition a bikini is two pieces) with two relatively wide straps that cover the nipples and form a deep, straight "V" cut all the way down to the pelvis. It doesn't do any other fancy designs or cuts. It's intended to look like a realistic swimsuit, i.e., something a woman might actually wear, not a piece of dental floss fantasy. It's trained and tested with photorealistic checkpoints, although I imagine it will also work with other styles but I haven't tried.

The model is trained on images of blonde, brunette, redhead, black, Asian, and Native American ethnicities and a wide range of breast sizes from A-cup to holy shit. However, fine-tuning breast size is hit or miss because it tends toward bigger, so you may find it helpful to use a breast size slider lora if you want to control that. It's not trained for nudity, although you might get it with the right prompts.

Notes and limitations:
With certain checkpoints, it has a tendency to want to add misplaced or malformed nipples inside the straps. This is improved in v2, but if it happens it can be stopped by adding "nipples, misplaced nipples, malformed nipples, nude" to the negative prompt.

It sometimes bleeds the swimsuit color into the girl's hair, especially with Asian and Native American ethnicities. This is improved in v2, but still happens occasionally. It can be stopped by adding "color in hair" to the negative prompt.

It leaks like all get-out, to the point that you don't even have to actually use the trigger word.