She started out as a mascot for a Nitroplus music event, and her popularity eventually led her to becoming the company's mascot in general. As for why she got so popular, it's...kind of hard to say. Her appearance obviously plays a big role, she has a lot of the whole 'endearingly dorky' thing going for her, and she kind of plays around with typical waifu bait cliches, too. It's actually kind of funny, but in a number of ways, she's...basically Honoka, but less annoying. And with
a much less grating voice.
They have similar appearances and mannerisms, though Sonico has more depth to her character as a result of being the focal point of both a game and an anime. They both can play guitar (
very well, at that). They both have interests that go against their stereotypically girly images (Honoka likes fighting and idolizes fighters, Sonico likes punk rock and at one point considers doing her hair spiky like the drummer in a band she likes). They were both raised by their grandmother, with their parents never really being mentioned at all. Both of them have taking hot baths listed as their hobbies... The list goes on. I'm like 90% sure Sonico was the biggest influence on TN making Honoka.
She's also cute as a button.