Libra's the Chief of the Minos Bureau of Crisis Control, he's not as high up as say the council: But he's pretty high up there, and the Sinners are all under him as well as his subordinates, Sinners included. If there is a Nightingale running around, would absolutely love to write things between him and his second-in-command. That aside, employees who deal with Sinners or the control room and etc. who are associated with the Bureau or even the FAC are welcome. Can also AU him into a business/modern verse where he is either a police Chief or a Chief Investigator, and your muse is under his command.
Another scenario I would love to write is an AU where Libra is a teacher at a university and the girl is his student.

A scenario where Libra is forced to fuck your muse for reasons beyond his control. I do NPC Parma who's a complete and utter bastard, maybe the man himself is railing your muse and says he'll leave them be if Libra completely breaks you. Maybe your muse is the one who's pushed him to fuck her senseless and is working with Parma for it, but Libra has no idea.

Would LOVE to play this in his PTN canon, but look: Libra's life is already hard as is, he constantly has Sinners and Corrupted trying to kill him, and the bureaucracy is always out for his life and his head on top of his attempts to deal with having no memories of who he was when he woke up in a cryostasis pod. The stress builds up over time, and quite a few Sinners just love to troll him and make things difficult for fun.
Maybe your muse caught him on a bad day when you decided to tease him, maybe your muse is just a consistent brat that loves to make his life hell at every opportune moment. Either way, he's had enough and if they're in the interrogation room: He tells Nightingale to turn off the microphones and camera before bending your muse over the table and fucking her senseless.

WHO DOESN'T LOVE A LITTLE MIND BREAKING.... Though I would absolutely love to see someone riding Libra stupid. He's by nature a top, and that won't change; So I'd love to see his partner really pushing him past his limit and riding him so much that his tolerance snaps and he proceeds to bounce the lady on his lap until she breaks. Added bonus would be cervix penetration.

Libra is very pent up and will absolutely go for several rounds once he is set off. He'll try to hold himself back because he is a considerate partner, but if he's fully set off and say....your muse pleads for him to knock her up, he's going to snap.

In Libra's care are multiple Sinners who are not entirely in their right mind due to Mania. While he's typically in control, there several circumstances where he's caught off-guard or the power he possesses does not affect the target/he's not able to reach them to do so. I'd seriously love to write a scenario where he's overpowered, restrained, and ridden/teased senseless and into submission, possibly to the point of finally giving in to the other's feelings.

...Okay look, I really don't have an explanation for this but LISTEN--

Some basic lore dump from PTN: The shackles, though they will only work on those afflicted with Mania and primarily those who are Sinners: Those afflicted with Libra's shackles, once activated, cannot disobey him. For example, when one of his Sinners was in harm's way in-game: He used the shackles to force her to dodge due to emergency, and her body moved against her will to bend at an odd angle to avoid the attack. Another ability he uses consistently, is that when he shackles a Sinner: He makes it so that they are unable to use their powers as a Sinner.
In that sense, he could most certainly force a shackled partner to keep cumming and cumming, or forbid them to reach their climax no matter how much he rails them. Or while he's teasing them to insanity, forbids them from moving a muscle to stop him.