fellow guest; fellow lodger; fellow passenger
Refers to someone sharing the same accommodation, inn, or vehicle as a fellow customer. Often used in travel or lodging contexts.
I chatted with a fellow guest at the inn.
On the night bus, the fellow passenger next to me slept the whole time.
Compound of 相 (あい, mutual) and 客 (きゃく, guest). The exact historical derivation is uncertain, but the meaning is transparent from the components.