i've been poking at this too long to care how visually appealing it is anymore. My headcanons for the main characters' sexualities, based on canon information! (has no bearing on ships.) Canon speculation below the cut.
marcille (bi): fascinated by romance and loves romance. we only see her show explicit interest in a male (fictional character (General Halleus from the book series she loves)), but i don't think she's fully straight.
falin (aro? ace? lesbian? genderqueer?): falin's only interest in relationships in canon is 'she considered accepting shuro's proposal because she was afraid nobody else would want her, but felt it'd be unfair to him because she had no feelings for him'. i consider whether she's aroace or a lesbian or maybe bi/pan, and she also seems like she might have some genderqueer feelings, based on some of her discomfort with her body and wearing certain types of femme clothing. (Also the fact that she‘s part male dragon.) Since she ends the story going on a journey for herself, it feels like she'll finally get the chance to figure out what she wants.
laios (pan, demi): he hasn't shown explicit interest in men, but similar to marcille, i don't feel he's fully straight. He’s aesthetically attracted to monsters, at the very least, so gender probably doesn’t factor in for him. romance/sex just don’t seem to be much of a high priority to him in general, but he did think his ex-fiance was cute and didn't seem uncomfortable with the idea of marriage (just seemed unhappy with being trapped in his hometown), so i feel like demisexuality fits him well.
About his succubus: He was very noticeably not stopped in his tracks by it like Chilchuck and Marcille, but that could possibly be because it just….looked exactly like Marcille, not an obvious fantasy. He started blushing and stammering heavily when it turned into a monster, which like….this boy is definitely a furry/monsterfucker, if anything, but that doesn’t speak on his attraction to actual humans.
I think it speaks for something that the succubi are able to literally read minds and craft the perfect fantasy for their specific target. And for Laios, it wasn’t just “his friend Marcille”. It was a version of his friend Marcille that wasn’t grossed out by monsters, didn’t think he was weird for wanting to be one, and was able to turn Laios into one. It was a Marcille who understood him at his deepest level that made him become a blushing, stammering mess to rival Chilchuck. Which is why I think he’s Demi, and needs a strong emotional connection with someone before he finds them attractive.
kabru (pan): his special interest is people, and he's bold enough with his sexuality to kiss rin despite not being in a relationship with her. so being pan/bi feels appropriate.
chilchuck (bi): he has a wife, and they were childhood friends, so he's definitely allo. but his comments and behavior towards senshi makes me suspect he might be bi, and just never considered the possibility due to being in a committed relationship.
senshi (gay, ace): this is 90% off of vibes. he keeps to himself in the dungeon and doesn't seem to have any need for social company, he's a complete hermit. Being ace makes sense to me, but so would him just having a low social drive. His succubus was 'a woman he hadn't seen since he was a child', but his journal implies it wasn't a romantic/sexual attraction.
namari (bi/lesbian): she is at the very least attracted to women, given her behavior with kiki, but she does make a point to say that kaka is also attractive to her, and her friends at the bar tease her about Kaka being her “new” boyfriend (implying previous boyfriends).
shuro: the token straight (in love with falin, asked her to marry him). i love you shuro <3 (but i can also see him being into men. there's no evidence to the contrary)
izutsumi: aroace. literally no question. her succubus is her mother.