Basics of Creating a Character

The basics of creating a character will usually involve the following information:
Name, Gender, Age (appearance if you like), Position in the manga, Occupation, Blood Type, Birthday, Favourite Colours/Foods/People/Books etc. A quote they live by, their motivations and their background.

The most important thing about a character is its personality. Only after you have that set can you begin the process of drafting etc etc. The best way to set personality is to know what your plot requirements are. If you haven't read the plot creation tutorials I recommend you read them now.

A few Conventions:
Most manga with a philosophical tone tends for a male character that is somewhat tortured and in some kind of mental instability. The female of such a manga (If there is one) will usually be either a peripheral presence that is completely ignorant of the male's feelings (Think Asuka, Evangelion) or an insightful one that is somehow mysterious and problematic in her own actions (Think Rei, Evangelion).

Most girl manga tend to have a female lead character that is rather happy and open, a very likable person if they were to be in our world. They would always have friends of course, at least one very good one, and a boy they either secretly admire or say they hate.

Most boy manga tends for a male lead character with a likable personality and mostly pretty 'good'. If you play DnD, they'd probably be Chaotic Good alignment. If there is a female lead the story is usually tending towards comedy or even hentai.

Names should be clear, concise, and most importantly, easy to remember. I recall the days of manga with lengthy English names published in Japanese, I had a lot of difficulty memorizing the names. Most manga artists these days prefer to use Japanese names. It's an easier way to give a name substance and meaning without being corny or lengthy about it. A good rule is to try keeping at least main character names to under 3 syllables.

Asian culture places great importance on horoscopes and blood types. Study the conventions for these and make sure you've got the right match.

O - Happy disposition, usually open and caring.
A - Elegant, naturally smooth, can get a little edgy if they didn't get their way.
B - Practical, open, strong-willed, intendant
AB - Silent, introverted, soft-spoken.

As for horoscopes, I recommend you get a small booklet from any naturopath store. Most bookshops also carry these.

Most younger audiences want characters their own age. Around 12-14. If your manga is more teenage geared you'll want to do the same to the characters, or somehow endear them to the older audience and the younger audience at the same time.

A philosophical manga that is trying to convey a deep message isn't going to do well with a younger appearing set of characters. And a comedy trying to appeal to the younger age group isn't going to get laughs talking about adult themes with adult characters.

Give the character a habit or a hobby, be it bad good or stupid. Having some kind of habit makes the character more realistic and lets the reader identify with them a lot more. It also gives the reader some grounding in the story, just that simple act could take them back and tell them 'relax, the climax is over for now'.

For example:
Rita (Sailor Jupiter) likes to cook, Daidouji Tomoyo likes to film Sakura and make up all these things for her. Chii says 'chi' a lot. And Yoh always looks too laid back for his own good.