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1. A succubus takes the form a person most wants a kiss from.

2. A person does not always know what form their succubus will take.

3. This is not necessarily a romantic or sexual kiss (Izutsumi's takes the form of her mother).

4. Succubi do not make mistakes. Whatever form they take is the form the person most want a kiss from. This only fails if there is some other factor that prevents the person from accepting the kiss.

From this, we know that the person Laios most wants a kiss from is Marcille. Laios, as shown in the panels, does not believe Marcille would ever kiss him, and in fact believes she would be horrified if he ever thought of her that way, and so the the succubus' ploy fails. Immediately after, the succubus switches tactics, concocting a scenario in which Laios could believe Marcille wants to kiss him, and this succeeds.

Now, people have argued that this does not represent Laios having romantic feelings for Marcille (see 3). Some go further to say that Laios, like Izutsumi, is asexual and so the succubus chose the form of a strong platonic affection. This ignores three key points.

1. The person Laios has the strongest non-romantic affection for is Falin. Like Izutsumi's succubus was her mother, Laios' succubus could have been his sister. Not only is it easier to believe Falin would give him a kiss on the cheek as soon as they're reunited, Falin's state as a chimera would also better lend her to the succubus' second tactic than Marcille.

2. Laios is sexually attracted to women. He makes this clear in the manga, when he expresses attraction to both orc women and elf women.

3. The succubus' form is not just Marcille, but it's Marcille with her hair down (like when she resurrected Falin, matching the shapeshifter Marcille) and smiling. Laios has been enamored with Marcille's smile since the day he met her.

Any explanation for why the succubus takes Marcille's form has to account for why it's Marcille, and why it isn't Falin. And any explanation that does so other than "Laios has romantic feelings for Marcille that he is repressing" is going to be a massive stretch in the face of the preponderance of evidence.