Like a lot of shoujo series, relationships take precedence in character development and plot. Even though Tactics beholds more of a supernatural drive and psychological feeling, relationships take great primacy, especially those concerning Kantarou.
All writings below are slightly opinionated considering they are based solely upon the way I viewed the series and character interactions. However, I take pride in presenting given facts instead of warping relationships to fit my own desires and likings. Also, if certain relationships present two viewable aspects, I will certainly express both points of view.
The moment Kantarou meets Ayame, Hatsumi's new assistant, he instantly blushes (in the anime). I have to admit it was quite cute, and it is evident he's attracted to the pretty girl. Though I think what most attracted him was her personality—the two act quite similar with their happy-go-lucky ways.
Even though he was attracted to her at first sight, for the remainder of their time together he treats her exactly as he treats anyone, and shows no further attraction or emotion towards her. (Perhaps he was momentarily stunned?) It almost struck me as if someone had flicked a switch.
Where it is true that Ayame is a fan of Kantarou's and idolizes him greatly, she seems to show no attraction towards him either. So yes, we are left with two pleasant strangers. Even though a relationship between the two may be considered "cute", due to their actions and lack of feeling, such would never work, or at least not in the intended setting of the series. And truthfully, the episode in which she appeared was more of a filler episode than anything, not entirely contributing to the main storyline.
This relationship is probably one of the most confrontational and over-disputed there is in the series. You'll always find that there are two protestant sides to this argument: The side that looks at them as nothing more than friends, and the side that sees their forbidden love. Where it is not my place to say which is right or wrong, I'll argue both points. Whatever the outcome... Well, regardless of what I say or think, it's still your personal decision to choose which form of their relationship takes precedence.
Standing on neutral ground, this relationship is one of the main focuses of the series. It is made quite clear in the beginning that humans and yokai simply don't mix. Romance, friendship—the two species are fated to never coexist unless they wish to endure emotional strife and ache. And yet we have Kantarou, a man striving to befriend and cherish the very creatures taboo to his fate. This factor creates a rift in the story, and gives it thorough plot. It places a fissure between Haruka and Kantarou, and one of the main plot focuses is the crossing of this border.
Haruka, regardless of what type of relationship, is an idol and physical morality for Kantarou. The only thing that got him through his childhood was the legend of the oni-kui tengu, a yokai far stronger than any other. He strove to find this tengu and break its seal to make himself stronger both mentally and physically in hopes of withstanding the cruelty of the world. That, at first, is his emotional attachment to Haruka, and soon he longs for that attachment to become mutual.
Coming from a friendship perspective, the two go from uncomfortable strangers to two souls striving to reclaim the familiarity they once possessed. Towards the end of the series, after they had withstood so much and finally came to accept each other's ways, Haruka's true self creates a rift between the two. After that, sorrow flies and hatred burns, demons of the past threatening to destroy the Haruka Kantarou strove to protect, and kill the boy Haruka came to accept as a friend over a master. In they end, thanks to a certain spirit, they are able to reclaim their common ground although acknowledging the fact they may end up killing each other, but that's the perfect compromise as long as they can remain together a little longer. (Very sweet if I do say so myself, and it takes a very strong bond for both parties to come to such a decision, placing their lives at risk to be with someone they adore and admire.)
Now, switching to a passionate point of view, the anime is full of shounen-ai fan fodder. From devilish comments to blushed faces, there are many hints of romantic feelings between the two. Kantarou becomes quite famous for whispering beautiful words such as, "I would die for you," time and time again. In addition, scenes such as the hot-spring, and when Kantarou jumps in front of Haruka during episode thirteen to stop a critical attack, almost killing himself (then held very sensually by Haruka) displays a sense of protection they hold over each other, and the hidden compassion within. And, of course, all fans of the couple remember the scene with them both in bed, injured, and Kantarou draped across Haruka's sleeping body. This relationship is bitterly sweet, and when it crumbles before Kantarou's eyes, you witness his sorrow and detachment held within. He becomes the ghost of a person—dead and unfeeling. Even despite death attempts held against him to release Haruka from his hold, he cannot due to the love he holds for the tengu, and he admits to that selfish attachment. The wording, too, can be interpreted many ways. For those who support one side or the other, it complies, but regardless of the personal outcome, these two characters are able to forge a bond stronger than fear itself, and that is something to be admired.
After her second husband died, Kyouko became a very wealthy woman, her named tied into a large sum of money. When meeting her, Kantarou spent the majority of his time flirting and complimenting her in the most obvious manner. He passed it off quite easily and convincingly that he was attempting to win her heart to inherit her fortune. (A plan most encouraged by Youko.)
Where it is quite evident he had no romantic feelings for Kyouko, his whole "marriage for money" front wasn't all that true either. Due to his current murder investigation at that time, he most-likely tried to warm up to Kyouko in hopes of finding a lead with his case. (She did become quite suspicious.) Also, do to the fact she wielded a great amount of power in her village, and that all the victims were tied to her, it was a logical decision.
What some people, however, may not realize is that Kantarou holds a lot of respect for her regardless of how well he shows it or not. Though I wouldn’t quite call them friends.
Mai was a girl consumed with bitter rage, living in her twin sister's shadow, and belittled by her mother. Under the assumption Kantarou was attempting to wed her mother in hopes of inheriting her family's fortune, she acted cold and spiteful towards him and the others. Though unlike everyone else who passed Mai off as a cynical brat, Kantarou was able to see through her porcelain mask and understand her actions and feelings. As he attempted to befriend her, pushing her insults and rash actions from his mind, she became frightened almost, receiving compassion for perhaps the first time in her life.
Mai, however, is a very driven girl. Once she sets her mind to or upon something, it is hard for her to falter. Still convinced Kantarou was after her mother, she attempted to seduce him with her body and guarantees that if he were to choose her he would still inherit the fortune. Kantarou, conversely, saw her hidden reasoning, and quite blatantly told her she was not the rebel she portrayed herself to be. Shocked by his compassion and refusal of her seduction, something snapped within and she began to feel desperate and beaten. Someone was finally showing her the empathy she always desired, and it frightened her.
When Mai was revealed as the culprit behind the supernatural killings, she explained that she was trying to gather enough life energy to save her sister, Miyabi, from death's grasp. Kantarou soon clarifies that she was being persuaded by a demon, not the guardian of Hell, and that what she was doing in no way aided her sister. Moments later, as Miyabi takes her last breath, Mai pleads with Kantarou to sacrifice her life to save Miyabi's, knowing he has the ability to do so. Kantarou, however, sadly refuses without explanation. With Miyabi's death, Mai curses him to his grave, saying forever she'll hold a grudge against him. He takes it in stride, and promptly leaves the village. What he refused to tell Mai, though, was that a sinner like her could never be sacrificed to save a life. He did this in hopes of fuelling her teachings and giving her reason to become a successful maiden and spell weaver like himself.
I want a Muu plushie, but that really has nothing to do with this section, so... (If you're amazing enough to make me one, I'll love you forever. *hint-cough*)
One of the biggest misunderstanding in this relationship is the idea that Muu actually loves Kantarou. (And I don't mean love in this society's generic sense. I mean the actual amour behind the word.) Where I will agree that Muu cares deeply for Kantarou's well-being, she does not possess romantic feelings for him. Infatuation, yes. Romantic love, no.
Her infatuation arose when Kantarou gave her mouth to mouth after she deflated and drowned. After that, she began to cling and follow him around, helping in her own special ways when she could. Her husband, Sugino, soon falls into the illusion that she loves Kantarou, but it is revealed she loves Sugino deeply, and even though she greatly likes and is infatuated with Kantarou, Sugino still means more to her. Muu is a wonderful friend, and a caring creature (in her own way), but not much more than that.
Kantarou and Sugino clearly have a hate-tolerate relationship, and it's more of a one-sided strife than anything. Where it is true Kantarou will occasionally taunt Sugino-sama, it is Sugino that holds all the cards of dislike and insult. This relationship might actually be interesting if both parties agreed to the conflict. Instead we are left with the ignorant Kantarou and the raging Sugino. And all you can think is, "Wow, it's like a chibi cartoon without the chibiness." (Or something among those lines, but they are unmistakably the comic relief.)
Unlike some of the characters in this series, Sugino is about as easy to read as a first grader's textbook. His reasons for hating Kantarou are as follows; his dislike for all humans (due to the fact they teased him many moons ago *cough—immaturegrudge*—cough*), Muu-chan (his wife) is evidently infatuated with Kantarou and clings to him like groupies to the Rolling Stones, the fact Kantarou was the human who broke Haruka's seal, and, last but not least, the disagreement between Kantarou and Sugino concerning Haruka's past. Where as the first two are quite self-explanatory, the first Haruka related issue is perhaps fueled by Sugino's dislike for humans, and the fact that a human such as Kantarou was able to break a powerful seal many before attempted to shatter. (Even Sugino-sama, I believe, though I may be incorrect.) In addition, the fact that Kantarou easily befriends and gains the trust of yokai is overly frustrating to the mountain god. (Seriously, he's just jealous and feeling inferior.) The second Haruka related issue involves conflicting emotions and reasoning. Sugino wants Kantarou to let Haruka remember his past and release his sealed strength. (Due to the fact Kantarou named him, Haruka is no longer the beast he once was.) Sugino understands Haruka's pain, thus fights for his freedom from Kantarou and the ability for the tengu to rediscover his forgotten past and strength. Kantarou, however, disagrees selfishly, longing to keep Haruka as he is now. (This notion can be explored more in Kantarou's relationship with Haruka.)
At the beginning of the series, it is quite evident Suzu-chan dislikes Kantarou, a slight dislike that grows and quiets down, finally disappearing with the course of time. And this, for the most part, was caused, progressed and regressed for many reasons. Though, first I will stress one point: Suzu may dislike Kantarou, but never did she truly hate him. (Common misconception.)
At the very beginning, Suzu accuses Kantarou of being a fraud, a user, and a con artist, attempting to steal her father's money (concerning his usage of paranormal activity and exorcism). This is, quite clearly, where the first hint of distain comes from. Though what set everything in motion was Kantarou releasing (the beautiful) Haruka, whom Suzu instantly developed feelings for. What continues to fuel Suzu's dislike of Kantarou for the longest time is the relationship between himself and Haruka (regardless of what way you interpret the relationship—romantic or friendship). Bluntly, it is distain fueled by jealousy. Though as time begins to pass, Suzu realizes and understands the compassion and strife Kantarou holds, and his righteous and spiritual ways. In the end she comes to respect him, and gives him one of the greatest gifts of genuine love.
Your first impression of Youko is that of perhaps a diligent wife to Kantarou, but with the passing of chapters or episodes, you can see those two possess the same bond as siblings. At times they can be quite alike, scheming together, tricking people, thinking of the next plan to make some money. Great minds think alike, perhaps? *cough*
Where some people would push this and try to apply a romantic relationship, it doesn't work and there's no supporting proof. Where it is true Youko is attentive towards Kantarou and greatly respects him, this is due to the fact he is her master and holds the power of name over her. The two have been together for a long time, and it evolved into a family of two, though in this case two children with no parents. And considering the two of them are always trying to wed the other off for money, that should be more than enough proof that their romantic love for green is much greater than their romantic love for each other.