I think this term could be found both in the otaku circles and in the general society, and its connotation might slightly differ between men and women, rather than these two societies.
If he is so mischievous and sometimes teases you to enjoy himself, he would be labelled as a 小悪魔系[koakumakei]. 悪魔[akuma] means a devil and 小[ko] means small or a bit. So literally, it means a baby satan(am I correct?). So I think this is easier to understand than ヒール系[hiirukei] or 腹黒[haraguro]. He shouldn’t be that evil, nor merely childish. Yes, 小[ko] makes things more lenient. In general, it could be said as ちょい[choi]. For examle, ちょいオタ[choi ota] means a light otaku (not so deep). Well, 小悪魔 might be equal to ちょいワル[choi waru]. ワル[悪] means evil.
If he is just childish and active, he would be labelled as やんちゃ系[yanchakei] or 元気系[genkikei] such as Amano Ginji in GetBackers or Hihara Kazuki in Kin-iro no Corda. But he has to be a bit evil…and surprises you all of a sudden in many ways…Yes, I can defintely think of Hitachiin Brothers in Ouran High-School Host Club, Hinoe in Harukanaru toki no naka de 3, or maybe…Asaba in His and Her Circumstance. Having said that, 小悪魔系 seems to need a shota-looking. Here, 小 still functions. He shouldn’t be a mature Satan, but immature, I suppose so.
On a side note in BL circles, when this type of guy is uke(bottom), he should be labelled as a 小悪魔受け[koakuma-uke]. Even though he has a shota-looking, he seems to control the seme(top). When they do it, he asks the seme whatever he likes. But at the last minute, he changes his mind and refuses to have a sex…A while ago, khursten and I settled Wolfram in Kyou kara maou as オレ様受け[ore-sama-uke], but would he be qualified to be a koakuma-uke?
A while ago, I encountered an interesting discussion here. The question is, how different is the general object of 小悪魔系 between men and women? What men feel about koakuma-kei and what women feel about it seem to be different somehow…Women would generally think a 小悪魔系 girl would cheat men and play around. And sometimes she might be even very mean. Well, it seems to be like a 女王様系[joousama-kei] such as 沢尻エリカ[Erika Sawajiri] if you ask me. Also, she might wear a fake fur or leopard-patterned cloth or something like that (like Ayumi Hamasaki[浜崎あゆみ]). Meanwhile, men would think a 小悪魔系 would do the same thing, but we barely know she is doing this on purpose or not. She might be innocent or evil, oh no, hard to tell. In anime, I can think of Misamisa in Death Note, and in real world, it might be ゆうこりん[Yuukorin] who got back from Korinsei[こりん星].
May I have your opinion? What comes to your mind first when it comes to 小悪魔系?