A long-standing member; a veteran or old-timer (someone who has been part of a group or organization for a long time).
彼はこの会社の古参社員だ。
He is a long-serving employee at this company.
古参の選手は試合の流れを作ることが多い。
Veteran players often set the pace of the game.
新参と古参の意見が対立して議論が白熱した。
Opinions between newcomers and veterans clashed, and the discussion got heated.