Aizawa Taki is a character from the story Gravitation, which is owned by Maki Murakami and Sony. He is by no means the actual villian of the story, more of a plot device, and all his actions (however cruel) are in the end, moot, and hardly even addressed in the storyline.

He's a total prima donna, a loud, spoiled, selfish brat. He's the lead singer in the band ASK, but he's not in charge of it. Ma-kun is, and Ken-chan is 'talent' -- think Kaikan Phrase. Taki's just the voice, the prettyboy they slap in the middle of the poster.

Eh? I thought it was "Tachi"?

"Tachi", is actually a nickname. You'll notice while reading the mangas and watching their brief time in the tv series, that only his fellow band members call him "Tachi". All official spellings of his first name are always "Taki", we soften it up by saying "Tachi" instead. Guys in Japan don't generally have nicknames when they're older -- so one could, in a rather sideways way, presume that the band ASK have all known each other for a long time, since they all know each other by nicknames. "Ma-kun" is obviously not his full name, and "Ken-chan" would be another nickname, since "-chan" is borderline offensive when applied to guys in Japan, unless you happen to be the guy's girlfriend. "-chan" is sort of a way of mocking guys, by way of appending an effeminate/childish/cute title to their name.