It denotes someone who, despite being a boy, has an appearance and personality that is undeniably that of a girl.

Even girls, as long as they use boku to refer to themselves and have a boyish appearance, can sometimes be treated as an otoko no ko in fanworks.

Oh!

Ah... But I refer to myself with boku, but I have a feeling that I'm not treated like that.

Eh?

That's not fair...