Remove the strings from the celery and slice it as thin as you can. Sprinkle with 1/4 tsp salt and let it sit for 5 minutes.
Cut the end off the chili pepper, shake out the seeds, and slice thinly.
In a bowl, whisk the rice vinegar, sugar, and remaining salt together until the sugar dissolves. Squeeze the water out of the celery and add it to the vinegar mixture. Add the sesame seeds and chili. Chill for 30 minutes.