2015–2023: Mainstream success
On August 14, 2015, Hikakin released "YouTube Theme Song" which he co-produced with his older brother, Seikin. From this point, the pair began to produce music as Hikakin & Seikin. "YouTube Theme Song" reached number 78 on the Billboard Japan Hot 100. As of 2024, it has surpassed 100 million views, becoming the most-viewed video on his channel "HikakinTV". In November of the same year, "HikakinTV" surpassed 2.57 million subscribers, making it the most-subscribed-to YouTube channel in Japan.
On December 22, 2017, Hikakin became the first YouTuber to appear on Music Station Super Live. On May 12, he worked with members of SMAP on Fuji Television's SMAPxSMAP. In 2019, Forbes Japan included him in its "Top Influencers 50" list.
On April 10, 2020, on his YouTube channel, Hikakin posted a video featuring a conversation with Tokyo Governor Yuriko Koike about the COVID-19 pandemic, in response to Hikakin's suggestion. This video was viewed more than seven million times within 24 hours of its release. Forbes Japan stated this video is "easy to spread" because it was not monetized and included no advertisements. In an interview with BuzzFeed Japan, he later stated he believed influencers should use their platforms to provide accurate information to young people and hoped his content would benefit others in the future. On December 8, 2020, the video was ranked number one in YouYube's official list of "Domestic Annual Top Trending Ranking".
His channel kept growing, reaching nine million subscribers on April 8, 2021. Hikakin set 10 million subscribers as his next goal. When his "HikakinTV" channel reached its 10th anniversary, he announced he would post 10 videos with extra effort, from August 1 to 20. The 10th video is the music video for Hikakin & Seikin's song "Fire", produced to commemorate the anniversary. This is the first Hikakin & Seikin music video to be released on "HikakinTV" since "YouTube Theme Song" in 2015. Hikakin changed his intro animation to one in which he encourages himself to reach his goal and celebrates HikakinTV's 10th anniversary.
At 12:11 on September 10, 2021, his YouTube channel "HikakinTV" surpassed 10 million subscribers. The same day, he made a live broadcast and cried at the moment he reached the milestone. In September 17 of the same year, Hikakin announced he would donate 10 yen (about US$0.06) for every additional subscriber for the rest of his life. For every additional million subscribers, he would donate 10 million yen (about US$64,000), saying: "I am able to earn money thanks to all of you who watch my videos. That's why I wanted to contribute to society, even if just a little."
On December 18, 2021, Hikakin launched an iPhone 13 giveaway campaign on X (formerly Twitter). Participants were required to follow his official account and retweet the relevant post; 103 iPhone 13 units, valued at 10 million yen, were offered as prizes. The post garnered over 1.75 million retweets. In a video uploaded on December 21, Hikakin noted this figure surpassed the 1.087 million retweets achieved by an anti-racism post from BTS using the hashtag "#StopAsianHate". According to data released by Twitter Japan on December 10, BTS's post previously held the record for the most retweets globally in 2021. Ultimately, this tweet reached 1.8 million retweets, becoming the eighth-most-retweeted tweet.
On August 7, 2022, Hikakin released a video of a large-scale project in which he rented out Fuji-Q Highland, titled "Catch the Popular One! Hikakin Tag 2022 Summer" (人気者を捕まえろ! ヒカキンおにごっこ 2022夏). This video is a tag project he planned and held with 130 million yen (about US$910,000) as a "dream video that he really wanted to shoot" after his channel surpassed 10 million subscribers. After ist release on YouTube, it set a record by ranking first overall on YouTube's trending rankings, receiving 280,000 simultaneous viewers and exceeding five million views in one day after its release.
On August 16, 2022, his channel "HikakinTV" reached 10 billion total views. On December 6, he was ranked first in the "Top Trending Videos of the Year in Japan" officially announced by YouTube. This marked the third-consecutive year in which he was ranked first in the "Top Trending Videos of the Year in Japan". He also ranked ninth in the gaming category.
In early 2023, he posted a video of his first visit in 20 years to his alma mater , Myōkōkōgen Minami Elementary School, which was scheduled to close due to its integration with Myōkōkōgen Elementary School. At the request of the principal, he was accompanied by his older brother Seikin, who also graduated from the school, and his childhood friend and classmate YouTuber Masuo, who accompanied Hikakin as surprise guests; they created a commemorative painting and sang the school song with the students.