Questions & Answers about Ο επικεφαλής είναι εδώ.
Ο is the masculine singular definite article, so here it means the.
Greek uses the definite article much more regularly than English. In a sentence like this, where you mean a specific person — the leader / the person in charge — the article is normally required:
Ο επικεφαλής είναι εδώ. = The person in charge is here.
Επικεφαλής means something like leader, chief, head, or person in charge, depending on context.
It can behave like:
- a noun: ο επικεφαλής = the leader
- or an adjective: for example, ο επικεφαλής μηχανικός = the chief engineer
In your sentence, it is being used as a noun.