Ask anyone about this term, or you will see what kind of person he/she is depending upon the answer. If you ask me, I will answer like below.
To be more simple, 兄貴[aniki] means a big brother, and sounds a bit cooler than お兄ちゃん[oni-chan]. However if you say 兄貴, it wouldn’t work that way depending upon a context or a situation.
I am an anime fan, so speaking of 兄貴, I would definitely think of 水木一郎[Ichiro Mizuki] who is best known for his work on theme songs for anime or tokusatsu. Although he is still active even now, he used to sing a lot of anime songs 1970 onwards such as Mazinger Z or Babel Ⅱ. He is really a passionate vocalist, and the way he sings beats your heart for sure. So most anime fans worship him as 兄貴 or アニソンの帝王[anison no teiou or Emperor of anime songs]. Some of the anime songs I used to love are sung by him like Pro Golfer Saru年は聞くなよ, so I do adore him as 兄貴. Come to think of it, there is one more 兄貴, he is 影山ヒロノブ[Hironobu Kageyama]. His most famous song is absolutely CHA-LA-HE-CHA-LA which is the OP theme song of Dragon Ball Z. His singing voice is very soulful and it sounds like flying in the sky. If I have to mention more, I would say きただにひろし[Hiroshi Kitadani] or 遠藤正明[Masaaki Endou]. They sang the theme song of One Piece and Aba-ranger(アバレンジャー) respectively. Anyway, if a person mentions above singers upon hearing 兄貴, he/she is an anime or tokusatsu fan.
Have you ever watched any yakuza(Japanese Mafia) films? In the yakuza’s society, 兄貴 is used to address a senior you work with (replacement of 先輩[sempai]). Usually, they work as a pair; one is younger, and the other is experienced. The younger calls the other 兄貴. It could be written in katakana, アニキ. Nah, it doesn’t mean he/she is that kind of person if the person answers like this. Rather, I guess this answer would come out the most because this connotation has been generalized.
One more. If a person answers like below, he(not she) would be possibly…
There is a shooting game named 超兄貴(メサイヤ)[chou-aniki-Messiah-]. See these two muscular guys? I don’t know why 超[chou] comes ahead of 兄貴, but well, maybe it might mean a more muscular guy since 超 means super- or hyper-. Yes, 兄貴 means such a muscular and manly man in the gay society. At the same time, 兄貴 could be an elder and experienced(yes, that way) like Abe-san. Also, there is another way to say such an 兄貴, that is ガチムチ[gachimuchi]. You can combine the two as ガチムチ兄貴[gachimuchi aniki]. Gachimuchi is more preferable to say among fujoshis although most BL fangirls do not like such a genre. I am pretty sure there are few fangirls who follow ガチムチ.
Why ガチムチ? I guess it could be divided into two parts, ガチ and ムチ. ガチ seems to come from ガッチリ[gacchiri] or hard, and ムチ[muchi] from ムッチリ[mucchiri] or muscular. We also say ムキムキ[mukimuki] to mean a beautiful muscle.
It seems like ガチムチ is preferable among gay people whereas it is less popular among fujoshis. In other words, there is a separate looking that is acceptable by each society. Look at below.
He is a Japanese pop singer, Ken Hirai[平井堅]. Which society do you think he is more preferable? The answer is, yes, gay society. Short hair, mustache, and a strong face. These things are must-have items(笑). A looking like him is so-called イカニモ系[ikanimo-kei]. I don’t know why it is written in katakana, but いかにも[ikanimo] is a kind of adverb and the meaning is…うーん、難しいな。It seems to be sth or it looks like sth if nothing else. Or, a person who has all the appearances of gay even if he is straight. 系[kei] is a type of person. So the full context is like いかにもゲイみたいな人[ikanimo gay mitaina hito] or a person who really looks like gay.
I already explained why most gay people dislike BL before, but this is a reason, too. So for fujoshis, イカニモ系 is not acceptable. Besides, there is a certain type called デブ專[debusen] or ポチャ專[pochasen]. Both mean a person who loves only fat people. Here is a line between fujoshis(and fudanshies) and gay people. When I was chatting with two fujoshi friends of mine…
a(me): Do you like 兄貴？
b(fujoshi 1): Absoultely not.
c(fujoshi 2): Me neither. Looks too sweaty, you know.
b: But a friend of mine follows ガチムチ. She has a minor taste, and she loves to see dark, handsome, muscular guys.
c: I don’t dislike such a muscular guy. Just seeing is okay, but I can’t use my yaoi filter with those guys. My eyes!
a: Then do you like イカニモ系?
b: Well…I don’t think so.
c: Like Ken Hirai? Oh his face is very strong! 濃い！
b: bangin, BL is BL, not gay, don’t you know that?
a: Y-Yes, I do know that. I learned from you girls.
c: You are not a type of person who gay people like.
a: Oh yeah?
b: Yeah, because you are too skinny.
c: And no mustache.
b: You are kind of ヤオイ体型[yaoi taikei].
a: What is it?
b: I made it. I mean, a person who seems to show in BL. Since there is アキバ体型[akiba taikei], I think it has to be a word like this.
c: Oh that’s well put. Any guys in BL have to be skinny, mustn’t be that muscular, but more feminine.
b: And nipples shouldn’t be large.
c: Shouldn’t be dark. So you meet these items!
a: Hey, come off it.
b: Someone like you are going to be an uke[bottom] after all. Whoever you pair up with.
a: This is sexual harassment! (((;゜д゜)))