member, a person who is in a group / participant, attendee at an event / (programming) member in object-oriented programming / (Internet slang) suspect in a crime

(engineering, computing) core / (only in compounds) core (being central, crucial)

(slang) hardcore, passionate

(sports) member of a starting lineup

(business) core competence

(only in compounds, music) hardcore

Demon core, a spherical subcritical mass of plutonium manufactured during World War II by the United States as a fissile core for an early atomic bomb

lurker (in an online community) (abbr. ROM, ロム)

happy hardcore (music genre)