If no subs exist:
The video in question is a classic of the "Doujin" (fan work) culture. It typically features the fan-character Tomari Mari, often animated in a rough but expressive style, set to high-energy or emotional Vocaloid music. shinseki nokotowo tomari dakar english sub work
The "work" resonates with fans because it takes the lore of Maria-sama ga Miteru (a series about an all-girls Catholic school) and adds a tragic layer. Tomari represents the student who never graduated—the one who "stayed" (Tomari). The lyrics often reflect themes of being left behind or moving on to a "New World" (Shinsekai), which fits the character perfectly. If no subs exist: The video in question
If you have the video file (e.g., an MP4), check its hash or filename on: Search using partial keywords: shinseki , tomari ,
Search using partial keywords: shinseki, tomari, overnight relative.