From Inkipedia, the Splatoon wiki

You★Me is one of the Squid Sisters' music producers, along with Shy-Ho-Shy. Although they have never been seen in the Splatoon series, they are mentioned in the interview with Marie, Marina, and Big Man in Splatune 3. In the interview, Big Man states that You★Me is a new producer who worked on Tomorrow's Nostalgia Today, with Marina noting the differences in the song compared to the Squid Sisters' previous works, including a "sense of maturity" that Marie states was added to reflect the gradual changes in Inkopolis since the debut of the Squid Sisters. According to Marina, You★Me's production style is characterized by a stronger EDM sound, featuring punchier kicks and supersaw-style synth bass.[1]

The name "You★Me" is a reference to composer Yumi Takahashi, who arranged City of Color (2023), Tomorrow's Nostalgia Today, and Liquid Sunshine.