Emi and Maou had a showdown, Emi won and Chiho appeared during this time with Emi's sword pointed at Maou's back. From a glance it looked like she was going to finish him, but in fact, she was actually removing his demonic powers to turn him into human.
Emi walks over to Chiho and tells her "don't use me and Alas Ramus as an excuse", suggesting to Chiho that she wants her to be with Maou, as Emi holds their friendship dearly and genuinely wanted her to be happy. Chiho and Maou talk and he professes his love for her. He tells her some BS reason explaning why he couldn't properly respond to her feelings before and the reason was because he was a demon but that isn't the case anymore, that although he's now just a poor, useless, weak human, he wants to protect her as he is, if she'll have him. Of course she accepts, stating that the person she fell in love with was always human in her eyes.
I'm going to add that Chiho kissed him previously in the LN, not sure which volume it happened in (probably 18 or 19) but yeah that time she spoke to him alone to ask a "reward" from him. She gave him a surprise kiss and slyly tells him that, that's only half the reward and she'll wait for him to initiate the next one. It looks like that could've been a foreshadowing because he does return the kiss to her in the final volume after all. :/
Oh and Chiho knew Emi was in love with Maou too and for some reason made it her business to let Maou know about it. I guess it's because she knew Emi wouldn't confess otherwise, but still I don't think it was necessary. It's unclear if Maou had any feelings for Emi, he didn't comment much on it, just that he wasn't expecting that. Get this though, Chiho told Maou that she wouldn't mind if Maou had other wives like demon lords in other worlds, but Maou rejected the idea due to money issues. Lame f*cking ending.