Ayaka doesn't appear until episode 4 of the TV Series. She's a lot like Shuichi in one way. SPOILER Ayaka is Yuki's fiance' but then she breaks it off. She becomes the next love interest of Hiro. SPOILED
Fujisaki is the Keyboardist to the band, Bad Luck. He's also only 16 years old I think. Fujisaki is dedicated to his art and expects nothing less than perfection from himself. SPOILER Fujisaki is the cousin of Seguchi Tohma, the keyboardist to Nittle Grasper. He also doesn't join Bad Luck until the third episode. SPOILED
Hiro is the guitarist of Bad Luck and Shuichi's best friend. They're practically like brothers, although Hiro is more patient than Shuichi. SPOILER Hiro was in love with Shuichi for a while, then he fell for Ayaka. SPOILED
K is Ryuichi's manager. K has a gun fetish, and hails from the USA. SPOILER K becomes Bad Luck's manager when Sakano becomes the producer. K also used to be a secret agent, which explains his love of guns and his skills. SPOILED
The band's, ASK, keyboardist. ASK is Bad Luck's rival.
Kitazawa, I can't give too much info without spoiling. In fact, I implore anyone reading this NOT to spoil themselves about this character, however I will still put a spoiler up. SPOILER MAJOR Kitazawa's full name is Yuki Kitazawa. He was Yuki's teacher when Yuki was a teen. Yuki fell in love with him, but ended up killing him. SPOILED MAJORLY
He's the guitarist for ASK.
Mika is a token female of the series. She appears in the first episode and almost appears in every episode after that. SPOILER At first, Shuichi thought she was Yuki's girlfriend, but she's his sister. She's also Seguchi Tohma's wife. SPOILED
One of the keyboardists to Nittle Grasper, she's usually seen around Ryuichi.
Known as Sakuma-san most of the time, he's the lead vocalist of Nittle Grasper. Ryuichi has two distinct personalities, his On-Stage serious guy one, and his off stage funny-as-hell crazy child-like one. Personally, I think both are the real him.
Sakano is the Manager of Bad Luck. He has a lot of freak-outs throughout the series. SPOILER He becomes Bad Luck's producer after a while. SPOILED
Shuichi is the lead vocalist of Bad Luck, and one of the LEAD characters of Gravitation. He admires Ryuichi like a god, and loves Yuki like one as well. Shuichi can be a bit childish at times but he has a good heart.
The lead singer of ASK, absolutely despises Shuichi. Slight Spoiler It's funny that throughout the whole series, we never once get to see or hear ASK play. Spoiled
Tatsuha is Yuki's younger brother, who is practically the Yin to Yuki's Yang. (Did I get that right?) He gets many laughs throughout the series, and has a major thing for Ryuichi.
Tohma is a keyboardist of Nittle Grasper and the President of NG Corporations, Bad Luck's music label. SPOILER He's married to Mika. SPOILED
Ah Yuki, the romance writer who Shuichi falls for. Yuki is so unlike Shuichi, in other words he can be a bit unpleasant at times, but he has his reasons.