Fujioka Haruhi is the main catalyst of the story. Tamaki was the last one to notice she was actually a girl, and ever since then his feelings towards her keep getting more contradictory and confusing for the boy. He insists he's "like a father" to her, though most of his actions say otherwise. Take a look at my Tamaki/Haruhi ship manifesto if you're interested.

Ootori Kyouya is Tamaki's best friend. He was the first person he got in contact with after moving to Japan and 3 years after there's no question the two of them would do anything for the other. Despite looking like quite the opposites, they are actually alike in some respects-- and really understanding of the other's feelings and situation.

Haninozuka "Honey" Mitsukuni is a year older than Tamaki, despite not looking like it. When he decided to open a Host Club, he contacted Honey and asked him to join it. At the time, Honey was captain of the Karate club and tried to act strict and tough to please his father. It was Tamaki who told him that living as someone he wasn't was also a form of cowardice.

Morinozuka "Mori" Takashi is the same age as Honey and also his cousin. The Morinozuka family had protected the Haninozuka family for generations, and despite becoming related to each other decades ago thus breaking the tradition, Mori still faithfully protects Honey. Tamaki also asked Mori to join the Host Club-- he probably knew he was essential to Honey's happiness.

Hitachiin Hikaru is the slightly older Hitachiin twin. Despite being virtually identical physically and terribly dependent of each other, the Hitachiins are of course different in their behaviors and ways of thinking. Tamaki approaches them perhaps being aware of how little and isolated their world was, causing them to be disdainful towards the rest of people.

Hitachiin Kaoru is the youngest, more mature and sensible of the twins. This is especially noticeable when he's by himself, since he tends to act calmer than his brother. Despite Tamaki not being able to tell between the two at the beginning, they still accept to join the Host Club since they found the boy "amusing" enough for them.