Short answers, pulled from the story.
Ecchi is a Japanese slang word meaning playfully sexual behavior or naughtiness. As an adjective it translates roughly as "sexy," "dirty," or "naughty"; as a verb it means "to have sex"; and as a noun it describes a person of lascivious behavior.
Ecchi refers to softcore or playful sexuality and does not imply perversion or fetishism. Hentai carries connotations of sexual deviance and perversion. In Japan the distinction is partly tonal; calling someone etchi can sound flirtatious, while calling them hentai sounds like condemnation.
Ecchi is an abbreviation of hentai, derived from the first letter H, which is pronounced "etchi" in Japanese. The word hentai itself was introduced during the Meiji period as a term for transformation or change of form in science and psychology, gradually shifting toward its sexual meaning over subsequent decades.
One of the earliest documented uses appears in the magazine Shukan Asahi in 1952, which reported a woman responding to being groped in a movie theater with the phrase "ara etchi yo," meaning roughly "hey, that's perverse."
Ecchi works commonly feature conversations with sexual references or innuendo, suggestive posing, revealing or sexualized clothing, nudity with genitalia obscured, and implications of offscreen sexual activity. Explicit sexual intercourse and visible genitalia are typically absent.
Ecchi themes are a specific subset of fan service, the broader category of content designed to please audiences rather than advance plot. Fan service covers any audience-pleasing element; ecchi narrows that specifically to sexual themes, often used for comedic effect in shonen, seinen, and harem works.