A regular customer or patron — someone who frequently visits the same shop or establishment.
あの寿司屋の常連になってから、店主と話すのが楽しい。
Ever since I became a regular at that sushi place, I enjoy chatting with the owner.
常連客には割引サービスがある。
There are discount services for regular customers.
彼はそのカフェの常連客で、毎朝来て新聞を読む。
He is a regular at that café and comes every morning to read the newspaper.
A constant companion — a person who is habitually together with someone, used sometimes figuratively.
あの常連たちは店の雰囲気を作る大切な存在だ。
Those regulars are important in creating the atmosphere of the shop.
彼はいつも同じメンバーと飲む、いわば常連だ。
He always drinks with the same group of people — in a sense, they're his regulars/companions.