Main Character Names:

ハルヒ = Haruhi
環 = Tamaki
鏡夜 = Kyoya (We'll use the Kyoya spelling from the Anime rather than Kyoyua to keep things simple)
= Hikaru
馨 = Kaoru
崇 = Takashi, but normally called by his nickname 'Mori'. (モリ)
光邦= Mitsukuni, but can be abbreviated to 'Honey'. (蜜 or ハニー)

Side Characters:

蘭花 = Ranka, the name Haruhi's father uses at work.
猫澤 = Nekozawa, the president of the Black Magic Club.
れんげ = Renge, the 'manager' of the Host Club.
希実子 = Kimiko, frequent visitor of the Host Club, but called by her last name 桜塚 (Sakurazuka)
百華 = Momoka, Class Vice-President of Haruhi's class. Often visits the Host Club.
She is still called by her last name, 倉賀野 (Kurakano) though.
上賀茂 = Kamigamo (つばき = Tsubaki) A regular of the Host Club.

New Characters:

レオ = Léo, a long-time French friend of Tamaki (original character). He moved from France to Japan to study but stayed in contact with Tamaki. He runs a rival Host Club at his own school, Kenran High. Part of the plot involves them trying to recruit Tamaki for their Host Club. He's sometimes called ジャン・ピエール・レオ (Jean Pierre Léo).
小百合 = Sayuri (Sayuri Himemiya), a childhood friend of Haruhi. Freshman at Ouran. (original character).
豪徳寺 = Gotokuji, another original character. Sophomore. Was raised in America, but moved to Japan. Another Kenran High Host Club member.
王 = Wan, the 4th and final original character. He's originally from Hong Kong but moved to Japan to study at Kerun High. 王龍(わんろん)(Ron Wan) is his full name. Some websites place his name as Wang Long. But we'll stick to the Ron Wan for the purposes for consistency.

Lesser Characters that do not have spirits in-game:

男子生徒 = Male Student
女子生徒 = Female Student
真嶋 = Majima. A side character whom is a student that on the 4th day in gameplay interacts with Sayuri. Will look into it further as don't know much about them. But that is the only day we encounter the character, so they don't seem very significant.

Bits to help:

ホスト部 = Host Club
先輩 = senpai, when referring to a senior. If a character is called Senpai at a point, e.g. 環先輩 - Tamaki-Senpai, please input the translation as Tamaki-Senpai.
お客様 = customer, one of the many girls/guests that frequently visit the host club.
みんな = Everyone
ああ = please translate as 'Yes', or a similar confirming response. Mori often answers in this manner.
「」= quotation marks (" ")

Honorofics

君 (くん) = -kun, mostly used when referring to a (younger) male or friend.
ちゃん = -chan, mostly used between female friends, or for youger kids. Honey tends to use these a lot to highlight his childish nature.
さん = -san, a polite way to address someone, equal to the English 'Mr.', 'Mrs.' and 'Miss', or if you're not acquainted with someone.
様 (さま) = -sama, used when the addressee is from a higher rank or out of admiration for someone.
(For example: Kamigamo calls Tamaki, 'Tamaki-sama'.)

Other notes:

These two things below are in-game pointers/functions. They are the point at which the game decides to award a point for a decision you have taken. So please ignore these two phrases:
ハルヒ指名率 = Haruhi's Customer Quota
グラフキャラ = Character graph/chart