Ichijou is a funny and sociable character, so he has a lot of relations with the other ones of the series.
Obviously, the most important is the one with Kaname, thet I explained in the proper section.

 

ICHIJOU & YUKI

Ichijou is always kind and caring with her. He's very close to Kaname, so he knows the truth about Yuki's identity of Clan Princess. But this isn't the reason why he likes her, he's kind with all the girls, and Yuki is kind with him too. They have a lot of meaning and he invited her to his Birthday Party. Yuki and him share a good relation, she respect and admire him, especially after a metting in which Ichijou explain to her that he could do whatever for Kaname. Yuki understands that they're close and Ichijou is a good friend, because even if Kaname have to deals of a lot of things, there is someone that helps him with all himself. Also, Ichijou saved her from a Level E Vampire, and in that moment Yuki have seen a different person. Not the usual kind one but a fighter with a sword, that menage it with ability and agility. Yuki was surprised but she always thought that he hides something behind his smile.
Yuki is called by him "Yuki-chan".

ICHIJOU & ZERO

We can't see a real relation between Ichijou and Zero, but in the Official Fan-Book we can read that Ichijou says that the person that he respect the most is Zero...??? There we can see that he estimes even his mother, but Zero is next her. It's very curious knowing that Ichijou estimes Zero. Maybe for his perfect work as guardian of the Accademy, or maybe because he's a safe person, but this is how we can know. O_o
Zero is called by him "Kiryuu-kun".

ICHIJOU & SHIKI

Ichijou is a sort of older brother for Shiki, and their relation is close and special (before Sara, damned her forever -_-), because Ichijou lets that Shiki drinks his blood (even hiding the fact in order to doesn't provoke the rebellion of the other normal vampires, that can't drink blood into the school) and many times we can see them together. Ichijou's family is involved in Riido's planes, so when Ichijou realizes that Riido was hide in Shiki's body, he tryis to save him in all the way, even deciding to face Kaname. Shiki likes him considering him the most important friend, but after Ichijou's departures after the battle against his grandfather, he doesn't see him again. Shiki and Rima find Ichijou's katana but not him. In a second moment they realize that Ichijou is alive and he's fine, next Sara Shirabuki. Shiki feels that he's different, he isn't the usual person I knew, but he can't make nothing in that moment, so he has a discussion with Rima about him. Ichijou is still the same or now he became the puppet of that bitch?
Shiki is called by him simply Shiki.

ICHIJOU & RIMA

Ichijou likes Rima. Since they share every vacations and every moment of freedom from the Accademy and the world of the humans, he likes to play, and sometimes he asks to Rima to play with him at the battle with the cuscions or hide and seek. Rima is bother by him and she doesn't agree, but she likes him as a good friend.
Rima is called by him just "Rima".

ICHIJOU & IDOU

Their relation isn't deep. As the vampire he is, (when he isn't busy to make Kaname's job) Ichijou spends his time together with the others of the group. He's kind and smiling with them but in the right moment he becomes cold and inflexible. This happens with Idou's "genial ideas", that always put the others in an awkward position. Idou doesn't like him so much (maybe because of the closeness to Kaname) but Ichijou treat with him like with the others.
Idou is called by him simply "Idou".

ICHIJOU & LUCA

As for Rma, Ichijou is kind with Luca. Since he's funny and sarcastic, he likes to make fun with her saying embarrassing things (as the time in which they're in vacation and he asked to her to sleep together XD, Luca's face became green and she refused the offer XD), that she always considers like the words of a child that want to children. But Luca knows how serious he's, and she respects him as he respects her.
Luca is called by him just "Luca".

ICHIJOU & AKATSUKI

The relation between Ichijou and Akatsuki isn't nothing important, they're friends like the others and they're part of the same group of vampires.
Akatsuki is called by him "Cain-kun".

ICHIJOU & ASATO

Asato is the one that grews up the little Takuma, teaching him what's respect and observance of the rules. In a first moment Ichijou is frightened by him, but when he understands the real plan of the grandfather, he decided to face him, even using his katana, the weapon that he always left for the battles against the Level E Vampires.
Ichijou is totally devoted to Kaname and he always stayed by his side, because the friend is more important than family and surely, that a old man that wants to destruct the peaceful convivence between humans and vampires.
We can assume that Asato is dead, because we can't see him anymore in the development of the story. So, if it's so, he's killed by Takuma.

ICHIJOU & SARA

I want to do a premise: I really hate Sara, so if you like her, DON'T READ my comments, ok? She's the most horrible and stupid character from the series and in every chapter I pray for her death! -_-
Why? She's a bitch, her only goal is be close with Kaname and realize the plan that Ichijou's family share with the Shirabuki's clan, the one to destruct the convivence between humans and vampires. She's a pureblood vampire (but her heands are dirty, we know! -.-) that saves Takuma from death after the fight against Asato. And in order to use him against Kaname, she maks him her toy. Ichijou is manipulated by her but at the party organized by Clan's family he confessess to Shiki and Rima that Sara isn't a bad person (???) and they're sure that he's in love with her LOL.
I can0t believe it, I DON'T WANT to believe it, because Sara is a bastard! Run away from her, Taku! =_=