Some examples could be series like 'To Love-Ru' or 'High School DxD' which have a lot of ecchi and borderline hentai content.
There are several manga that could be considered close to hentai, like some adult-oriented ones with explicit content. But it really depends on your personal definition and tolerance for such content.
Basically hentai manga is mainly characterized by its bold and graphic depictions of sexual situations. It goes beyond the usual boundaries of typical manga in terms of content. However, it's important to note that such manga is often restricted to adult audiences due to its mature nature.
The key features could include well-defined characters with their own motivations and goals. Also, a coherent plot structure that builds tension and leads to a satisfying conclusion. Additionally, it might incorporate elements of surprise or twists to keep the readers engaged.
Hentai adventure manga often features explicit sexual content and fantastical storylines. The characters may have exaggerated traits and the settings can be quite imaginative.
Glasses hentai manga often features characters with glasses in sensual or explicit situations. The art style might focus on emphasizing the allure of the glasses-wearing characters.
I think the main features could be the unusual and potentially controversial storyline involving a babysitter in a hentai context.
Hentai manga gagged usually involves elements of restraint and submission, often with characters being silenced or restricted in some way.