In Naruto Gaiden: The Seventh Hokage manga, page 3
Who is the guy with blue hair sitting next to Boruto?
Is it a new character or did I miss something from Naruto?
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The character sitting next to Boruto is Mitsuki. He is a Konohagakure shinobi who originated from an unknown village and also the team-mate of Boruto and Sarada.
According to Narutopedia:
Born as Orochimaru's son, Mitsuki immigrated to Konohagakure, where he was then enrolled into the Konoha Academy. Most of Mitsuki's background is left as a complete mystery to the point where even his own team-mates don't know who his parents are.
Extract from Boruto: Naruto the movie summary:
Boruto questions Mitsuki about his parentage, to which the latter says he is the son of Orochimaru. This shocks Sarada and she asks if Orochimaru is his mother or father while Boruto asks who Orochimaru is, and Mitsuki replies it doesn't matter, much to Boruto and Sarada's confusion.