From xkazemg:

Tango was betrothed to Ako when they were both just seven years old. Later, Tango was killed in a car accident, but his parents survived. In the other car, both parents died but Rukuruu survived... Eventually, Tango's parents adopted Rukuruu and he became Ako’s fiance in Tango’s place.

Ako is annoyed by Rukuruu's apathy - is he not concerned at all that he's living Tango's life...!?