Why does Okabe Rintarou (Steins; Gate) refer to himself as Hououin Kyouma?

Okabe, as a self-proclaimed "mad scientist", has a bit of an eccentric personality. As a result, he uses the name Hououin Kyouma (鳳凰院凶真). As he explains:
鳳凰 (houou) means phoenix
院 (in) means temple / institution
凶 (kyou) means evil
真 (ma) means truth

So a translation of his nickname would be "Phoenix temple, evil truth" - it's a ridiculous name, but to him it sounds cool for a mad scientist.

to answer why, you should just watch Steins;Gate:Fuka Ryōiki no Déjà vu. This explains all unanswered questions in the anime and especially yours, which is unanswered even in the VN.

the movie messes up one thing, how did Okabe choose the name Kyoma in the main world line for himself if he was not told by Kurasu?

They explained it in the VN; he becomes 'Hououin Kyouma' for Mayuri's sake. In the start of chapter 6 (I think), he saw Mayuri 'floating' towards the light (Angel's ladder) and in response, he then ran towards Mayuri and clung onto her, saying things about being a mad scientist and that Mayuri is his hostage. (In Okabe's thoughts, he explained this; he said that the mad scientist thing was from a show he and Mayuri used to watch, and that he continued talking about it so that he wouldn't feel embarrassed) So that's the start of Hououin Kyouma. The Movie is just an add on for the story; it doesn't explain anything. Cause in the VN/Series, Kurisu didn't interfere with the whole thing yet (becoming the observer to save Okabe). It was after Okabe reached the Steins;Gate worldline that Kurisu did what she did in the movie.

omggg the anime made me cry to death
Okabe is a mad scientist
Hououin Kyouma
鳳凰 (houou) means bird
凶 (kyou) means evil
真 (ma) means truth

So Bird evil truth
Or even, Thruthful evil bird
yeah ._. It's silly but so is he so yup

It's just a fake name which has "cooler" kanji in it, whose meaning he likes better. At one point he says:

"If you don't know, then memorize it. Ho-oh as in phoenix, In, the temple. Finally, the Kyou in atrocious becomes Ma in truth. Hououin Kyouma...!"

"rintarou" means something along the lines of "ethical plump son."

@M

Okabe Rintarou

Lmao

there is a movie which is a direct sequel to the anime that explains why he does it