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Gura's design isn't even eye catching compared to every other vtuber. She's just a plain anime girl with white hair and shark teeth (tons of chubas have that so it's not even unique). And doesn't wear the iconic hoodie. Gura is also the most COVERED UP vtuber girl at least in her default state. She has bad rigging. She's not the life of the party type girl, instead she's noticeably scared to say certain things and there's a lot of dead air, more than most vtubers. She's not ambitious and is a doormat for everyone except Shion who she out-bratted on stream. She is not skilled at games except rhythm games. She's not known for drawing skills and even though she's good at singing by untrained standards, her only original song doesn't showcase that talent. Her true singing talent only comes up mostly when she sings American oldies and certain classics.
Gura's success is mostly through clips, like anon has pointed out. But to be fair to be her, many of those clips are pretty entertaining which is why they spawn so much fan material. I do like that she featured her fans on that new song cover and they got to be on a song about a loli talking to lolicons.