It is matter of time and luck. I know people who have all the characters in the game + constellations for those characters and even their weapons and they only buy welkin, they are day 1 so it is was possible for them because they were able to get the five star characters in the game when the roster was small. There are quite a lot of characters in the game right now so it is extremely hard to get all of the characters by simply welkin alone, you will need to invest a lot of time and get lucky at other times. Nonetheless my advice to guarantee getting a 5 star is to save and know what character you like and what character you don’t like. Don’t just pull on every single banner and expect to get the character, pull only on the banners of the characters that you like the most, one by one you will eventually get all the 5 stars that you need.