Update:
My nephew and his family (#A) were flying, but the ticket was purchased by his father-in-law (#B) who used an Indian travel agency (#C).
So "C" paid the airline, and was reimbursed by "B" but the passenger is "A".

In this situation, where someone else has paid the airline, who should file the EU261 claim?
The passenger (A) or the person who ultimately was financially responsible (B) or the travel agent (C) who actually interacted with the airline and paid for the ticket?

Complicating matters a bit, what if B (and, hence C) is not interested in pursuing any claims?
("I do not trust the process, not worth my time" etc. is the response of B).
Can A still file a claim because he was the passenger who was inconvenienced?

Just in case it is necessary to add more flavor:
A is a resident of Sweden, B (and C) are Indian residents and citizens, and the airline is German.