A runner; a person who runs, often referring to an athlete in races such as marathons or sprints. Can also be used figuratively for someone leading something.
彼女は毎朝公園でランナーと一緒に走っている。
She runs every morning in the park together with other runners.
そのランナーはマラソン大会で優勝した経験がある。
That runner has experience winning a marathon race.
新製品の市場では、彼がランナーとなって先頭を切っている。
In the new product market, he is acting as the runner, taking the lead.