Depends on the MC, of course . Some MC may enjoy the chase and challenge, but others may want something that feels more simple and stable, which isn’t something that tsundere romances offer all that easily.
Example potential romantic interest in order from most to least for Lupin and his reasons (because why not):
Nat: Her calm, friendly nature puts him at ease and makes him feel comfortable. While Lupin is friendly, he can be rather slow when it comes to actually trusting others. But Nat’s willingness to work alongside him and the simple fact that she puts forth that effort to try and get along has made it easier the detective to let a good deal of his guard down. He rarely bonds with people so smoothly or quickly, but Nat has been one of the few in the group who tries to include him in certain discussions and makes it easy for him to offer just as much of a open hand.
Manson: More of a case of opposites attracting, where Lupin’s empathetic, gentle nature toward others is somewhat of a compliment to Manson’s more closed-off and harsher demeanor to outsiders. They both clearly don’t agree with each other on a lot of things, which leads to lots of snarking at each other. But for Lupin, Manson isn’t intrusive in the same way as Ava tends to be for him, so it’s not as bad; it simply feels like interacting with an equal who has a different approach to life.
Felix: While Felix is the one he is closest to in terms of friendship, a romance seems unlikely at the moment. He doesn’t take Felix’s flirting very seriously and simply sees it as his friend’s way of playing around (and other times completely misses the fact that Felix is being flirty). That said, he does enjoy the youngest vampire’s company and feels just as comfortable around him as he does with Nat. Seeing that Felix is new to a number of things, he’d probably be the first person Lupin would think about taking with him when out to do something new and fun as a means to relax. IF a romance was ever going to happen between them, Felix would have to try a different approach on the matter (or at least state interest in a more sincere manner to get Lupin to believe him).
Ava: I’ve seen a bucket of oil and water getting better acquainted than these two have been so far. Lupin tried to be cordial during the introductions, but Ava was clearly unwilling to meet him halfway. And as someone who cares about his hometown and is really determined to catch the killer to prevent more murders, her remarks and unwillingness to try and work together most of the time tends to trigger him (A LOT). It’s like when they take two steps forward and seem to muster up a desire to compromise and take a chance at friendship, Ava inadvertently does something that causes Lupin to immediately take five steps away from her, glaring a bit at her as he reminds himself that those two things are just not possible with her. Maybe they will eventual become friends who respect each other, but that’s as far as their relationship will ever go.
I have never loved a vampire bite joke more than when I first found this picture:
Though honestly, that joke sounds like it’d be just as funny in a number of situations, whether the Detective knows that they vampires at that moment or not.