There is a kanji character for “1” that looks like this: 一. However, in Japanese, there are vowels that need to be held when you speak. That is not related to numbers at all. The mark they use after their words that need to be held longer when they speak is just like an accent mark from Spanish or some other language. You put the accent mark (‘) on the park of the word in Spanish that needs to have more emphasis than the rest of the word. It often can change the meaning of the word. In Japanese, if you write something in hiragana, you can see if there is a vowel that needs to be held longer because they put a う (u) after it, but if it is transcribed into romaji, then they put the mark: —. This means that that the word has more emphasis on that vowel and must be held longer.
Here is a good example for you: 東京 (Tokyo) in hiragana looks like this: とうきよう. You can see that there are two hiragana that are う in that word. To express that when they write it in romaji (Roman letters), they write “to—kiyo—.” Those marks are for pronunciation. The have nothing to do with numbers. You actually pronounce “Tokyo” in Japanese much differently than how you pronounce it in English. In English, we say it quickly, “Tokyo.” However, in Japanese, those vowels are held longer and it comes out more like “to-u-ukyo-u-u.
If they write this in hiragana or kanji, they often write it from top to bottom, it doesn’t look like the kanji for “1” (-) anymore. It becomes a line like this: “ㅣ” just telling them to hold that vowel when they say it.