First before getting into their interactions I will go over the first mention of Sari. During the last chapter of arc 3 Rona is reporting to the Demon King about Raido being the Devil. Sari asks if she can be present during the meeting as she is interested in both his company and his personal strength. The Kuzunoha Company is strange because after a year and a half after being founded it is a stable economic powerhouse, it has a numerous military powerhouses, and finally even without outside help it seems to be strangely self-sufficient. Following this is a brief character-trait introduction for her as being the youngest among the Devil candidates and inferior in diplomacy but strong in handling information. The final bit of information we get on her is that he primary interest in the Kuzunoha Company is not for its warfare capabilities, but rather for its potential to help the demon race after the war is over.
The first meeting of Sari and Raido is never directly shown. After arriving in the Demon City and immediately having an audience where he greets the Demon King the scene cuts to after their initial meeting. The Demon King, two of his generals (I think?), and the four Demon King candidates were discussing their opinions of him. The majority consensus being that he was a threat. Due to Raido's hardline insistence on total neutrality and his immense economic and military capabilities if they accidentally upset him or some such they could easily end up being greatly harmed if not destroyed (one of the Princes compares it to living with an uncaged meat eating beast). The one dissenting opinion came from one of the generals stating that his potential threat is completely outweighed by the immense economic boon he would bring with him even if they only used his ability to transport goods quickly and safely. The Demon King then asks Sari for her opinion. She believes that both sides are right. The Kuzunoha Company and Raido possesses unchecked and seemingly unlimited power, but so long as it isn't turned on them even simply being present would greatly strengthen the nation. She basically says they are and magic bullet that could solve all their problems at the risk of blowing up in their face and killing them all. Thus she presents her solution, it would be fine if this meat eating beast just had a collar as insurance, so this magic bullet even if it explodes it hurts them only to a non-fatal degree
The next segment is when the two Demon Princess Sari, and Lucia were guiding Mio, Shiki, and Raido around the city with the primary goal being visiting the Spirits temple. When they first see the two Princesses, Raido is shocked by how young Sari is. He describes her as being about the same in age-wise appearance as Rinon (Remember her? Toa's little sister, estimated at 10-12 years old). Some stuff happens, Mio pisses Lucia off (the demon princess who is best at military affairs, also she is Io's direct disciple) by basically saying that she is only at the level of playing house. Throughout Sari is occasionally trying to mediate, but is primarily observing Raido's reactions (She think he doesn't notice, but oh he does). They reach the temple which is surrounded by a distortion and Sari asks if Raido and company could help investigate. The two princess promise to protect the group, but they really need to deal with the situation and fast. Raido's group agrees and they enter.
Lucia and Sari are slowing fighting their way further inward without the support of Raido and company (they are only following), but slowly they start to get overwhelmed by the sheer number of spirits. After Sari pleads with them a bit to help them retreat and Raido, after some brief resistance by Mio, instead decides at this point to just go for the whole thing and reach the altar at the core of the temple (its underground in case you were wondering). So Mio and Shiki substituted in and just start sweeping away the mobs of spirits and they briskly march onwards much to the shock of Lucia and Sari. As they start getting closer to the depths of temple without trouble Shiki notices and then warns that there are two High Spirits at the core of temple. Raido says its fine and that Shiki and Mio doesn't have to worry about really hiding their powers since the Demon race already sorta knows (They learned the hard way during the second invasion of Limia). Sari starts a mental conversation with Lucia that Raido listens in on. Since Lucia was rather disbelieving that Raido was strong since he didn't exude and aura of confidence that she had been taught all warriors have, especially strong ones (again she is the disiple of Io and the one who is probably gonna take over the military in the future), Sari starts off with saying how this was the true power of the Kuzunoha Company as they had three beings that seemed to have superpowers and that was just the ones that they know of and that the words sense of crisis doesn't even seem to come into Raido and the gangs mind even though they are going to soon encounter two High Spirits. Lucia still doesn't think Raido is personally that strong yet though as he hasn't been participating in the fighting at all. After a little bit more back and forth (that I completely don't understand) Sari starts talking about how since if they aren't likely to be selected as the next Demon King then they should start considering the other uses of women such as political marriages (By the way this is all happening inside a mind chat that Raido is using his world sphere power thing to listen in on). Lucia then asks Sari if her father (the Demon King) had approved of offering her to Raido as a marriage partner which according to Sari is on hold something that worries Raido as he was absolutely not a lolicon. Lucia tries to dissuade Sari from putting herself on the spot to for political marriage as she thinks that Sari is more important to the country and if any of the princesses has to do it she (Lucia) should since Io could more easily groom another future military head than Rona could groom a future head of information like Sari. They finally meet the spirits, battle starts, two princesses being in the standard shock over how OP Shiki and Mio and Raido are with how they are easily beating around beings who could turn entire cities to ash. Raido then decides that the two High Spirits would serve as good training partners for Mio and Shiki and steps back to let them handle them. The POV changes to Sari and we get the standard omg these people are strong as she describes their fight (like how Jin described the example fight between Makoto and Shiki). Shiki starts being pushed back and Sari starts panicking and wondering why Raido refuses to step in and help him. Shiki overcomes it though and says that he hadn't perfectly mastered the control of the 9th ring so he didn't want to use it but it couldn't be helped. He then summons these dimensional chain things that restrain the Earth High Spirit and then beats the living daylights out of it. Mio is fighting the Fire High Spirit (a phoenix ;D), she mostly doesn't block the phoenix's overwhelming attacks and her outfit is being shredded, but she doesn't seem to be actually taking damage and even scarier is how Mio and Shiki (aside from that initial moment) don't seem to be struggling at all. Of course all of this shocks our poor Sari and she comes to the conclusion that fighting the Kuzunoha Company would be like fighting the Goddess and so she resolves herself to no matter what get the Kuzunoha Company on the side of the Demon Race so that they reap the absolutely amazing benefits of being their allies.
The next chapter is a the report of Sari and Lucia and a discussion with the Demon King, Rona, and Io about what they saw. After seeing their powers Lucia is now on board with Sari and that the demon race needs some sort of way of keeping Raido in check. Fighting him is useless so they need a way to keep him from ever turning on the Demon Racs, the first step being allying and supporting him. Sari then starts pushing that she marries Raido as the method. The Demon King Zeph (finally found his name >.<) astutely notes that it is a dangerous suggestion as it will in all likelihood greatly piss Mio off to try to marry Raido and Sari. After some debate Sari asks permission to at least try and that she has already started preparing various methods to attach herself to him.
Zeph tries bringing it up with Raido at the party celebrating the swift conclusion of the incident by asking if he would like to marry either of the two of them (Lucia or Sari). Raido firmly refuses and the Zeph instead changes tactics and starts giving him lots of alcohol while offering sales pitches for Lucia and Sari. Since Raido remained uninterested Zeph asked what his taste in women was and if he likes Mio (Mio reacts by stretching herself to get as close as she can to hear his answer). For those interested his answer what that Mio is like a goofy older sister in a real family member sense and therefore he can't start thinking of her that way.
The final section happens after the Makoto vs all the Demon Generals friendly duel, and then preventing Root from obliterating the Demon Race.
After that crisis Zeph forces the title Hero of the Demon Race on Raido with a smile and spreads it throughout the country (Roots attack would have obliterated most of the country so he did have the right to say Raido saved everyone) and thus just like Hibiki is seen as the hero by the humans the Demons recognize Raido as their hero even though he hasn't accepted that position. Sari is with Lucia looking out the window watching all Raido's well wishers and newfound fans. Lucia announces (this section is confusing I think this is what she's doing) that she thinks marrying Raido would be a good idea and thus would try to win him over and Sari tells her that he refused to marry both of them. The logical course of action then is of course to….make yourself his slave by means of the ancient demon ritual (this is again different from the pact that he made with Mio, Shiki, Tomoe folks. It’s an actual magic spell that the Demon Race has passed down with its primary use being for the punishment of criminals). Lucia notes that it’s a very forceful method and that Sari acts fast (the contract mark is already visible as something like a faint bruise on her chest). You need some body part and apparently she took one of his gray hairs for that requirement since it was easy to take without him noticing. Also Lucia notes that it's too late for Sari to be convinced otherwise, since the contract mark is already visible it means it's already in effect and while the Demons created it they have never managed to remove it ever. Her first goal is apparently to gain his trust so she can start working towards getting a 'physical connection' with him.
After waking up Raido having accomplished his goals and being surrounded by fans when he went on a walk (not really but that sorta sums up the situation) starts preparing to leave. During his final audience with Zeph he is feeling immensely grateful that Zeph isn't pushing for Lucia's and Sari's marriage to him and is about to leave when Zeph tells him that the demon race has one last gift for him. Zeph calls Sari forward and Makoto (I can't keep calling him Raido its killing me) fearfully calls out her name ("….Sari-dono?" lol) because….. she is dressed in a cute maid outfit. Makoto engages in brief escapism while wondering why she is wearing that (maybe its pajama day lololol) when Sari comes in front of him and swears fealty to him as her now eternal master. Chaos ensues with Mio saying this girl has done something she shouldnt have and will immediately get rid of her. Sari asks Mio to wait and starts explaining while getting down on one knee in front of Makoto. She exposes her chest revealing her contract mark (with Makoto briefly noting she was a cutting board chest) and explains that the contract is an ancient Demon Race ritual that guarantees that his orders are always followed no matter the distance so that he won't have to fear her leaking any of his secrets to the Demon Race. Makoto asks about what if he releases her and she said it's not possible and trying to remove the contract mark will destroy her soul and that this contract is only used generally on heinous felons. Makoto huddles with Mio and Shiki and asks if Shiki could remove the contract and he says he probably could given 10 years to study it. Sari then says to them that is Makoto doesn't need her she might as well die right now and pulls out a knife and presses it against her throat. Makoto looks over to Zeph wondering why he hasn't stopped her, the reason being he says is that this was her decision that she didn't consult with anyone about and that she has lost all standing current and future with the Demon Race. That being said Zeph and Lucia is glaring at him which he complains about in his mind as she didn't even ask for permission to become his slave or do the ritual thing. Zeph comes forward and walks Raido off to the side a bit and starts a mind chat with him, the first words sentences being about how Sari is his real daughter from one of his mistresses and that despite being King he does view her like a father properly should. Furthermore since Sari really wants to serve alongside him he asks that as a father granting the selfishness of his daughter could Raido please accept her then Zeph walks away. Makoto is cornered and gives in. Mio likens it to Makoto being kind and picking up a stray dog. Shiki recommends that they place her under Eva's command in Keryuneon but Makoto decides to send her to Asora.
Thus is the story of how Makoto picked up a stray 'dog' or rather a slave maid Demon Princess on their fun journeys.